THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 


GIFT  OF 

Estate  of 
Ernst  and  Eleanor 
van  Lt5ben  Sels 


(/' 


/ 


M- 


LOAN  STACK 
GIFT 


J?76 


PREFACE. 


^  I  ^HE  present  work  has  its  origin  in  an  attempt  to 
comply  with  a  suggestion  which  has  frequently  been 
made  to  me,  that  I  should  prepare  an  abridged  edition  of  my 
translation  of  Ganot's  Elements  de  Physique,  which  could  be 
used  for  purposes  of  more  elementary  instruction  than  that 
work,  and  in  which  the  use  of  mathematical  formulae  would 
be  dispensed  with.  But  I  soon  found  that  to  do  anything  of 
the  kind  which  would  be  more  than  a  mere  series  of  extracts 
would  be  very  difficult,  and  hence  I  turned  my  attention  to 
another  book  by  the  same  author,  which  has  had  a  very 
extensive  circulation  in  France,  his  Cours  elementaire  de  Phy- 
sique, and  this  I  have  taken  as  the  basis  of  the  present  book. 

It  is  not  a  mere  translation,  but  such  additions  and  altera- 
tions have  been  made  as  I  thought  fitted  to  render  the  book 

318 


vi  Preface. 

useful  to  the  classes  for  which  it  was  more  especially  de- 
signed— namely,  as  a  text-book  of  physics  for  the  middle  and 
upper  classes  of  boys'  and  of  girls'  schools,  and  as  a  familiar 
account  of  physical  phenomena  and  laws  for  the  general 
reader.  In  range  it  may  perhaps  be  nearly  taken  to  represent 
the  amount  of  knowledge  required  for  the  matriculation 
examination  of  the  London  University. 

Although  English  scientific  literature  is  not  wanting  in 
works  in  which  the  main  physical  phenomena  are  explained 
in  familiar  language,  they  are  for  the  most  part — whether 
from  too  much  conciseness  in  some  parts  or  from  too  minute 
details  in  others,  or  again  as  being  too  costly — not  suited  for 
direct  teaching  purposes. 

To  facilitate  reference,  the  articles  of  the  present  work 
have  been  numbered,  and  a  copious  index  has  been  drawn 
up  in  accordance  with  this  arrangement. 

E.  ATKINSON. 

STAFF  COLLEGE: 

1872. 


ADVERTISEMENT 


TO 


THE      SECOND      EDITION. 


T  N  a  work  which  only  proposes  to  serve  as  an  introduction 
to  the  study  of  a  science,  no  great  additions  can  be 
made  without  exceeding  the  range  within  which  it  is  limited. 
Hence  in  the  present  edition  I  have  not  thought  it  desir- 
able to  add  more  than  about  twenty  pages  of  new  matter, 
together  with  twenty-four  new  woodcuts  and  a  coloured 
plate. 


E.  A. 


STAFF  COLLEGE: 
June  1875. 


PLATES. 


ELECTRICAL  DISCHARGE   IN   HIGHLY  RAREFIED  GASES  .  Frontispiece 


SPECTRA    OF    METALS    &c page  355 


CONTENTS. 


BOOK   I. 

GENERAL  PROPERTIES  OF  MATTER,  AND  UNIVERSAL 
ATTRACTION. 

CHA1-.  PAGE 

I.  PRELIMINARY  NOTIONS          .      .  .           .            .        ^•._,i 

II.  GENERAL  PROPERTIES  OF  BODIES  >    -                                 4 

III.  MOTION  AND  FORCE  .           .  .          V          *  .12 

IV.  MOTION  AND  FORCE  (continued)  .  .         •  ...     29 

V.     LAWS    OF    FALLING    BODIES.      INCLINED    PLANE.      THE 

PENDULUM  .  .  .  »  .'  '         -44 

VI.     MOLECULAR  ATTRACTION  ...  .  •    .     56 

VII.     PROPERTIES  SPECIAL  TO  SOLIDS        .  .  .  .62 


x  Contents. 

BOOK   II. 

HYDROSTATICS. 

CHAP.  PAGH 

I.     PRESSURES  TRANSMITTED  AND  EXERTED  BY  LIQUIDS  .      .     65 
II.     EQUILIBRIUM  OF  LIQUIDS     .  *   .  -74 

III.     PRESSURES  SUPPORTED  BY  BODIES  IMMERSED  IN  LIQUIDS. 

SPECIFIC  GRAVITIES.     AREOMETERS      .  .  83 

BOOK     III. 

ON    GASES. 

I.  PROPERTIES  OF  GASES.     ATMOSPHERE.     BAROMETERS 

II.  MEASUREMENT  OF  THE  ELASTIC  FORCE  OF  GASES 

III.  APPARATUS  FOUNDED  ON  THE  PROPERTIES  OF  AIR 

IV.  PRESSURE  ON  BODIES  IN  AIR.     BALLOONS 

BOOK   IV. 

ACOUSTICS. 

I.     PRODUCTION,  PROPAGATION,  AND  REFLECTION  OF  SOUND     150 
II.     MUSICAL  SOUND.     PHYSICAL  THEORY  OF  Music  .  .     163 

III.     TRANSVERSE  VIBRATIONS  OF  STRINGS.     STRINGED  INSTRU- 
MENTS         .  .  '      "'•'*,?  ''      ''"*"*        .  .     175 

IV.     SOUND  PIPES  AND  WIND  INSTRUMENTS     .  179 


Contents.  xi 

BOOK   V. 

HEAT. 

CHAP.  PAGE 

I.   .GENERAL  EFFECTS  OF  HEAT.     THERMOMETERS        ,  >',,    .     189 
II.     RADIATION  OF  HEAT  .  .        fo'irtj        *  '     2O2 

III.  REFLECTION    OF   HEAT.      REFLECTING,    ABSORBING,    AND 

EMISSIVE  POWERS    .        .    r  •         •  •  »      •     204 

IV.  CONDUCTING  POWER  OF  BODIES      .  .  .  .213 

V.  MEASUREMENT  OF  THE  EXPANSION  O*F  SOLIDS,  LIQUIDS, 

AND  GASES  .  .        •«.'•'  .  .      .     217 

VI.  CHANGES    OF   STATE   OF    BODIES    BY   THE   ACTION    OF 

HEAT    .  .  .  .  .  ,  .      225 

VII.     FORMATION    OF   VAPOURS.      MEASUREMENT    OF    THEIR 

ELASTIC  FORCE       .  .  ,"  .  .     231 

VIII.  LIQUEFACTION  OF  VAPOURS  AND  GASES  .            .  246 

IX.'  SPECIFIC  HEAT.     CALORIMETRY         .  /           .      .  252 

X.  STEAM  ENGINES      .            .            .            .  '         .            .  256 

XI.  HYGROMETRY    .            .            .            .  .                   .  270 

XII.     METEOROLOGICAL    PHENOMENA    WHICH    DEPEND    UPON 

HEAT      .  274 

XIII.     SOURCES  OF  HEAT  AND  COLD  .  .  .  287 


xii  Contents. 


BOOK    VI. 

ON   LIGHT. 

CHAP.  PAGi 

I.     TRANSMISSION,  VELOCITY,  AND  INTENSITY  OF  LIGHT    .     294 
II.     REFLECTION  OF  LIGHT.     MIRRORS     .  ;  .       .     302 

III.  REFRACTION  OF  LIGHT    ;  .  .          ;  .  i~  •        .     322 

IV.  EFFECTS  OF  REFRACTION  THROUGH  PRISMS  AND  THROUGH 

LENSES  .  .  ,  .          ':,  j*     ' .^'"^        .     320 

V.     DECOMPOSITION  OF  LIGHT  BY  PRISMS          ••;   •     -.--<>•  -     •     34^ 

VI.     INJURIOUS  EFFECTS  OF    COLOUR  IN   LENSES.      ACHRO- 
MATISM       .         .  .,  •:       -:v  '.        .        <',        f'^-..     .     361 

VII.     OPTICAL  INSTRUMENTS       .  ".*'          /         7 . 

VIII.     OPTICAL  RECREATIONS 


BOOK   VII. 

ON    MAGNETISM. 

I.     PROPERTIES  OF  MAGNETS     .  .  .  .  .400 

II.     TERRESTRIAL  MAGNETISM.      COMPASSES          .  ..40^ 

III.     METHODS  OF  MAGNETISATION          .        ^  ^         .,  .     411 


Contents,  xiii 
BOOK   VIII. 

FRICTIONAL    ELECTRICITY. 

CHAP.  PAGE 

I.     FUNDAMENTAL  PRINCIPLES            .            ,                        .  415 

II.  ACTIG&T  OF  ELECTRIFIED  BODIES  ON  BODIES  IN  THE 
NATURAL  STATE.  INDUCED  ELECTRICITY.  ELEC- 
TRICAL MACHINES  .  .  .  /  .  427 

III.  ELECTRICAL  EXPERIMENTS            .            .            .'        .  437 

IV.  CONDENSATION  OF  ELECTRICITY         .            .            .       .  445 
V.     VARIOUS  EFFECTS  OF  ACCUMULATED  ELECTRICITY         .  454 

VI.     ATMOSPHERIC  ELECTRICITY.    THUNDER  AND  LIGHTNING  461 

VII.     ELECTRICITY    DUE    TO    CHEMICAL    ACTION.      VOLTAIC 

BATTERY             .            .            .            .            .            .  470 

VIII.     EFFECTS  OF  THE  BATTERY      .            .            .            .  483 

IX.     ELECTROMAGNETISM            .....  493 

X.     ELECTRODYNAMICS       .          • .            .            .            .  499 

XL  ELECTROMAGNETS.  TELEGRAPHS  AND  ELECTROMAG- 
NETIC MOTORS  ......  504 

XII.     INDUCTION  BY  CURRENTS       .            .            .            .       .  519 

XIII.     THERMO-ELECTRIC  CURRENTS       ....  528 

INDEX 532 


ELEMENTARY     COURSE 

OF 

PHYSICS. 

BOOK   I. 

GENERAL    PROPERTIES    OF    MATTER,    AND 
UNIVERSAL   ATTRACTION. 


CHAPTER    I. 
PRELIMINARY   NOTIONS. 

i.  Definition  of  physics. — The  word  physics  is  derived  from 
the  Greek  (/.vote,  nature ;  for  the  ancients  understood  by  the  term 
physics  the  study  of  the  whole  of  nature.  They  comprised  within 
the  domain  of  this  science  mechanics,  astronomy,  chemistry,  botany, 
zoology,  medicine,  and  even  astrology  and  divination,  whether  by 
the  stars  or  by  the  observation  of  physiognomy. 

The  province  of  physics  is  at  present  much  more  restricted.  Its 
object  may  be  considered  to  be  the  study  of  those  phenomena  which 
do  not  depend  on  changes  in  the  composition  of  bodies ;  for  these 
belong  to  chemistry. 

Thus,  when  water  by  cooling  is  changed  into  ice,  and  by  heat 
this  ice  is  again  changed  into  water,  the  liquid  is  exactly  the  same  as 
before  ;  not  merely  are  all  its  properties  the  same,  but  its  substance 
is  identical  with  what  it  originally  was.  The  passage  of  water  to 
the  state  of  ice,  and  the  return  of  the  latter  to  the  liquid  state,  are 
physical  phenomena.  In  like  manner,  when  a  brittle  object,  of 
porcelain  or  glass,  for  instance,  falls  to  the  ground  and  breaks,  each 
piece  retains  exactly  the  same  chemical  composition.  The  fall  of 
the  vessel  and  its  fracture  against  the  ground  are  then  physical 
phenomena. 

On  the  other  hand,  when  wood  burns,  its  substance  is  profoundly 

B 


2       Properties  of  Matter  and  Universal  Attraction.     [1- 

modified.  It  consists  of  several  different  forms  of  matter,  and  is 
decomposed  ;  one  part  of  its  elements  passes  into  the  atmosphere 
as  smoke,  while  another  is  left  as  a  residue  consisting  of  ash  and 
charcoal.  In  short,  the  substance  we  know  as  wood  has  disappeared, 
and  is  replaced  by  others  which  are  entirely  different.  The  com- 
bustion of  wood  is  then  a  chemical  phenomenon. 

2.  Matter,  mass,  density. — We  understand  by  the  term  matter 
whatever  can  affect  one  or  more  of  our  senses  ;  that  is  to  say,  any- 
thing whose  existence  can  be  recognised  by  the  sight,  touch,  taste, 
smell,  or  hearing. 

The  mass  of  a  body  is  the  quantity  of  matter  contained  in  this 
body.  Different  substances  may  contain  very  different  quantities 
of  matter  in  the  same  volume.  It  will  subsequently  be  shown,  for 
instance,  that,  for  equal  volumes,  lead  contains  nearly  eleven  times 
as  much  matter  as  water,  and  gold  nineteen  times  as  much.  This 
is  expressed  by  saying  that  the  masses  of  lead  and  of  gold,  are 
respectively  eleven  and  nineteen  times  as  dense  as  water.  When  one 
body  has,  for  the  same  volume,  twice  or  thrice  the  mass  of  another, 
it  is  said  to  be  twice  or  thrice  as  dense ;  and  the  density  of  one  sub- 
stance in  reference  to  another  is  the  number  which  expresses  how 
much  matter  the  first  body  contains  as  compared  with  the  second. 

3.  Simple  and  compound  substances. — It  has  been  ascertained 
that  all  the  various  forms  of  matter  with  which  we  are  acquainted 
may  be  resolved  into  about  sixty-five  different  kinds,  which  are  called 
simple  substances  or  elements,  to  express  that  each  only  contains 
one  kind  of  matter.     Many  of  these  are  very  rare,  and  are  found  in 
very  minute  quantities  ;  others  are  more  widely  diffused,  and  have 
important  uses,  but  are  not  abundant ;  and  the  great  mass  of  the 
universe  is  made  up  of  about  fourteen  ;    the  non-metallic  bodies,  or 
metalloids,  oxygen,  hydrogen,    nitrogen,    silicon,  carbon,  sulphur, 
phosphorus,  and  chlorine  ;  and  the  metals  aluminium,  potassium, 
sodium,  calcium,  magnesium,  and  iron. 

Very  few  of  these  elements  occur  in  nature  in  the  free  state  ;  by 
far  the  greater  number  of  the  substances  we  know  are  compound ; 
that  is,  formed  by  the  union  of  two,  three,  or  four  of  these  elements. 
Thus  water  consists  of  hydrogen  and  oxygen  ;  marble,  of  carbon, 
oxygen,  and  calcium  ;  muscular  tissue,  of  carbon,  hydrogen,  oxygen, 
and  nitrogen.  The  number  of  substances  containing  more  than 
four  elements  is  very  small. 

The  force  in  virtue  of  which  different  substances  unite  to  form 
compounds,  and  which  opposes  the  resolution  of  compounds  into 
their  elements,  is  called  the  force  of  chemical  attraction  or  affinity. 


-5]  Different  States  of  Matter.  3 

4.  Internal  constitution  of  bodies.  Atoms, molecules,  mole- 
cular forces. — The  properties  of  bodies  prove  that  they  are  not 
formed  of  continuous  and  compact  matter  as  they  seem  to  be,  but 
that  they  are  agglomerations  of  excessively  small  material  particles, 
which  are  called  atoms.  The  elementary  atoms  can  unite  with 
each  other  to  form  compounds,  but  cannot  be  destroyed  by  any 
known  process. 

The  term  molecule  is  given  to  the  smallest  cluster  of  atoms  of 
any  substance  which  is  conceived  capable  of  existing  by  itself  ; 
every  pure  substance  consists  of  similar  molecules. 

The  same  properties  which  have  led  physicists  to  assume  the  ex- 
istence of  atoms  and  molecules  have  also  led  to  the  assumption  that 
these  small  particles  do  not  touch,  but  are  simply  juxtaposed,  retain- 
ing between  them  excessively  small  intervals,  which  we  shall  after- 
wards become  acquainted  with  under  the  name  of  pores  (9). 

But  it  maybe  asked,  How  is  it  that  bodies  do  not  spontaneously 
fall  into  powder  ?  What  gives  them  solidity  and  hardness  ?  What 
is  the  invisible  force  that  unites  atoms  and  molecules  ? 

This  force  is  the  reciprocal  attraction  which  the  molecules  of 
bodies  exert  upon  each  other  and  which  is  continually  drawing  them 
together.  The  force  which  holds  together  particles  of  the  same 
kind  of  matter  is  called  molecular  attraction ;  the  force  which  holds 
together  particles  of  different  kinds  of  matter  is  called  chemical 
attraction  or  affinity.  When  hydrogen  and  oxygen  unite  to  form 
water,  they  do  so  by  reason  of  the  exercise  of  the  latter  force,  while 
the  particles  of  water  are  held  together  by  molecular  attraction. 

If  molecular  attraction  were  the  only  force  acting  upon  the  small 
particles  of  which  bodies  are  composed,  they  would  come  into  com- 
plete'contact,  which  is  never  the  case.  They  are  also  under  the 
influence  of  a  repulsive  force,  in  virtue  of  which  their  particles  con- 
tinually tend  to  separate  themselves  ;  this  is  the  force  of  heat. 
Experiment  shows,  in  fact,  that  whenever  a  body  is  heated,  its 
volume  increases  because  its  molecules  are  driven  apart ;  while  on 
the  contrary  its  volume  diminishes  when  it  is  cooled,  because  the 
molecules  then  become  closer.  The  particular  form  which  matter 
assumes — whether  solid,  liquid,  or  gaseous- — depends  on  the  extent 
to  which  it  is  influenced  by  these  antagonistic  forces. 

5.  Different  states  of  matter. — All  different  substances  present 
characters  in  virtue  of  which  they  may  be  divided  into  three 
distinct  classes,  solids •,  liquids,  and  gases. 

Solids,  such  as  wood,  stones,  metals,  &c.,  are  substances  which 

B  2 


4       Properties  of  Matter  and  Universal  Attraction.     [5- 

are  more  or  less  hard,  and  retain  the  form  which  they  possess 
naturally,  or  which  has  been  given  them  by  art.  It  is  assumed 
that  in  solids  molecular  attraction  preponderates  over  repulsion. 

Liquids,  such  as  water,  oil,  meicury,  are  bodies  which  have  no 
hardness,  and  present  but  little  resistance  when  a  body  is  immersed 
in  them  ;  they  have  no  shape  of  their  own,  but  at  once  take  that  of 
the  vessels  in  which  they  are  contained  ;  they  are  virtually  incom- 
pressible. It  is  assumed  that  in  them  molecular  attraction  is 
balanced  by  the  repulsive  force  of  heat,  and,  that  while  the  mole- 
cules can  freely  glide  over  each  other,  they  keep  an  invariable  dis- 
tance apart  if  the  temperature  be  not  altered. 

Gases,  such  as  hydrogen,  oxygen,  carbonic  acid,  are  also  called 
aeriform  fluids,  from  their  analogy  with  our  air,  which  is  a  mixture 
of  oxygen  and  nitrogen.  They  are  very  light  bodies  ;  excepting  a 
small  number,  which  are  coloured,  they  are  invisible ;  and  hence  a 
vessel  filled  with  air,  hydrogen,  or  any  colourless  gas,  appears  quite 
empty.  Like  liquids,  they  have  no  shape  of  their  own,  but,  unlike 
liquids,  they  are  eminently  compressible  and  expansible.  In  them 
the  repulsive  force  of  heat  preponderates  over  molecular  attraction 
(4)  ;  whence  it  follows  that  they  are  continually  tending  to  occupy  a 
larger  space.  This  property  will  be  described  as  the  expansibility 
of  gases  (i  TO). 

There  are  many  bodies  which  can  exist  in  these  three  different 
forms  ;  thus  water,  exposed  to  great  cold,  becomes  solid  in  the  form 
of  ice;  at  ordinary  temperatures  it  is  liquid,  while  at  higher  tem- 
peratures it  becomes  a  gas.  Sulphur,  iodine,  and  several  metals 
present  the  same  phenomena. 


CHAPTER   II. 

GENERAL   PROPERTIES   OF   BODIES. 

6.  Extension. — By general propertiesvtz  understand  thosewhich 
are  common  to  all  bodies,  whether  solids,  liquids,  or  gases  ;  such  for 
instance  are  extension,  impenetrability,  divisibility,  porosity,  com- 
pressibility,  elasticity,  inertia,  and  gravity. 

Specific  properties  are  such  as  we -observe  only  in  certain  bodies, 
or  in  certain  states  of  those  bodies  ;  solidity,  fluidity,  tenacity,  malle- 
ability, colour,  hardness,  etc.  are -properties  of  this  class. 

The  first  general  property  of  bodies  with  which  we  are  concerned 


-8]  Divisibility.  5 

is  their  extension  or  magnitude  ;  that  is,  the  extent  of  space  they 
occupy.  All  bodies,  even  the  smallest  atoms,  have  a  certain  ex- 
tension. 

Extension  considered  in  only  one  direction,  that  of  length,  gives 
a  line  ;  in  two  directions,  length  and  breadth,  a  surface ;  and,  in  the , 
three  directions,  length,  breadth,  and  thickness,  a  volume. 

With  respect  to  the  above  general  properties,  it  may  be  re- 
marked that  impenetrability  and  extension  might  be  more  aptly 
termed  essential  attributes  of  matter,  since  they  suffice  to  define  it ; 
and  that  divisibility,  porosity,  compressibility,  and  elasticity  do  not 
apply  to  atoms,  but  only  to  bodies  or  aggregates  of  atoms. 

7.  Impenetrability. — This  is   the  property  in  virtue  of  which 
two  portions  of  matter  cannot  simultaneously  occupy  the  same  por- 
tion of  space.     Strictly  speaking,  this  property  only  applies  to  the 
atoms  of  bodies. 

In  many  phenomena  bodies  appear  to  penetrate  each  other. 
Thus,  if  a  pint  of  water  and  a  pint  of  alcohol  be  mixed,  the  volume 
of  the  mixture  is  less  than  two  pints.  A  similar  contraction  occurs 
in  the  formation  of  certain  alloys  ;  thus  brass,  which  is  an  alloy  of 
copper  and  zinc,  occupies  a  less  volume  than  the  united  volumes  of 
its  constituents. 

This  penetration  is,  however,  only  apparent,  and  is  due  to  an 
alteration  in  the  position  of  the  molecules ;  they  come  nearer  each 
other,  and  the  space  occupied  by  the  pores  is  diminished. 

A  nail  driven  into  wood  is  not  a  case  of  penetration.  The  mole- 
cules of  the  latter  are  driven  apart  by  the  nail,  but  wherever  it  has 
penetrated  there  is  no  wood.  When  water  has  been  poured  upon 
a  heap  of  sand  it  at  once  disappears  ;  the  water,  however,  does  not 
penetrate  the  substance  of  the  sand  itself,  but  merely  fills  the  in- 
terstices between  the  grains. 

8.  Divisibility- — This  is  the  property  which  all  bodies  have  of 
being  divided  into  distinct  parts. 

Numerous  examples  may  be  cited  of  the  extreme  divisibility  of 
matter.  The  tenth  part  of  a  grain  of  musk  will  continue  for  years 
to  fill  a  room  with  its  odoriferous  particles,  and  at  the  end  of  that 
time  will  scarcely  be  diminished  in  weight. 

A  piece  of  carmine  not  larger  than  a  grain  of  corn,  gives  a  dis- 
tinct colour  to  two  gallons  of  water,  from  which  it  can  be  deduced 
that  this  small  quantity  of  colouring  matter  cannot  contain  less  than 
ten  million  particles. 

Blood  is  composed  of  red,  flattened  globules  floating  in  a  colour- 


6       Properties  of  Matter  and  Universal  Attraction.     [8- 

less  liquid  called  serum.  In  man  the  diameter  of  one  of  these 
globules  is  less  than  the  3,5ooth  part  of  an  inch,  and  the  drop  of 
blood  which  might  be  suspended  from  the  point  of  a  needle  would 
contain  about  a  million  of  globules. 

Again,  the  microscope  has  disclosed  to  us  the  existence  of  in- 
sects smaller  even  than  these  particles  of  blood ;  the  struggle  for 
existence  reaches  even  to  these  little  creatures,  for  they  devour  still 
smaller  ones.  If  blood  runs  in  the  veins  of  these  devoured  ones,  how 
infinitesimal  must  be  the  magnitude  of  its  component -globules  ? 

Has  then  the  divisibility  of  matter  no  limit?  Although  experi- 
ment fails  to  determine  such  limit,  many  facts  in  chemistry,  such 

as  the  invariability  in  the  relative 
weights  of  the  elements  which 
combine  with  each  other,  would 
lead  us  to  believe  that  a  limit  does 
exist.  It  is  on  this  account  that 
bodies  are  conceived  to  be  com- 
posed of  extremely  minute  and  in- 
divisible parts  called  atoms  (4). 

9.  Porosity.  —  Pores  are  the 
extremely  small  intervals  which 
exist  between  the  molecules  of 
bodies,  and  porosity  is  the  property 
which  bodies  possess  of  having 
pores. 

Two  kinds  of  pores  may  be 
distinguished  :  physical  or  inter- 
molecular  pores,  where  the  inter- 
stices are  so  small  that  the  mole- 
cules remain  within  the  sphere  of 
each  other's  attracting  or  repelling' 
forces  ;  and  sensible  pores,  or  actual 
cavities,  across  which  these  mole- 
cular forces  cannot  act. 

The  contractions  and  expan- 
sions resulting  from  variations  of 
temperature  are  due  to  the  exis- 
tence of  physical  pores  ;  whilst  in 
the  organic  world  the  sensible  pores 
are  the  seat  of  the  phenomena  of  exhalation  and  absorption. 

In  wood,  sponge,  pumice  stone,  and  in  animal  and  vegetable 


-10]  Porosity.  7 

tissues,  the  sensible  pores  are  apparent :  physical  pores  never  are 
seen.  Yet  since  the  volume  of  every  body  may  be  diminished, 
we  conclude  that  all  possess  physical  pores. 

The  existence  of  sensible  pores  may  be  shown  by  the  following 
experiment  : — A  long  glass  tube,  A  (fig.  i)  is  provided  with  a  brass 
cup,  ;«,  at  the  top,  and  a  brass  foot  made  to  screw  on  to  the  plate 
of  an  air-pump.  The  bottom  of  the  cup  consists  of  a  thick  piece 
of  leather.  After  pouring  mercury  into  the  cup  so  as  entirely  to 
cover  the  leather,  the  air-pump  is  worked,  and  a  partial  vacuum 
produced  in  the  tube.  By  so  doing,  a  shower  of  mercury  is  at  once 
produced  within  the  tube,  for  the  atmospheric  pressure  on  the 
mercury  forces  that  liquid  through  the  pores  of  the  leather.  In  the 
same  manner  water  or  mercury  may  be  forced  through  the  pores 
of  wood,  by  replacing  the  leather  in  the  above  experiment  by  a  disc 
of  wood  cut  perpendicular  to  the  fibres. 

When  a  piece  of  chalk  is  thrown  into  water,  air-bubbles  at  once 
rise  to  the  surface,  in  consequence  of  the  air  in  the  pores  of  the 
chalk  being  expelled  by  the  water.  The  chalk  will  be  found  to  be  • 
heavier  after  immersion  than  it  was  before,  and  from  the  increase 
of  its  weight  the  volume  of  its  pores  may  be  easily  determined. 

The  porosity  of  gold  was  demonstrated  by  the  celebrated  Floren- 
tine experiment  made  in  i66r.  Some  academicians  at  Florence, 
wishing  to  try  whether  water  was  compressible,  filled  a  thin  globe 
of  gold  with  that  liquid,  and,  after  carefully  closing  the  orifice 
hermetically,  they  exposed  the  globe  to  pressure  with  a  view  of 
altering  its  form,  well  knowing  that  any  alteration  in  form  must  be 
accompanied  by  a  diminution  in  volume.  The  consequence  was, 
that  the  water  forced  its  way  through  the  pores  of  the  gold,  and 
stood  on  the  outside  of  the  globe  like  dew.  This  experiment  has 
since  been  repeated  with  globes  of  other  metals,  and  like  results 
obtained. 

The  Florentine  academicians  had  concluded  from  their  experi- 
ments that  liquids  were  incompressible  ;  that  is,  could  not  be  reduced 
in  volume  by  pressure.  This,  however,  is  not  the  case ;  liquids  are 
compressible,  though  to  a  very  small  extent  (74).  By  cooling,  a  far 
greater  diminution  in  volume  can  be  produced. 

From  these  facts  we  conclude  that  the  molecules  of  liquids  may 
be  brought  nearer  each  other,  and  therefore  that  there  are  pores 
between  them.  The  facility,  moreover,  with  which  liquids  mix  is 
a  proof  of  their  porosity. 

10.  Applications   of  porosity. — The  property  of  porosity  is 


8      Properties  of  Matter  and  Universal  A  ttr action.    [10- 


frequently  utilised,  more  especially  in  the  process  of filtration.  This 
consists  in  clarifying  liquids  by  freeing  them  from  particles  ot 
matter  which  they  hold  in  suspension  ;  as  is  done,  for  instance,  with 
river  water,  which  is  turbid,  owing  to  the  earthy  matter  it  carries 
along  with  it. 

The  apparatus  used  for  this  purpose  are  called  filters,  and  are 
usually  constructed  of  unsized  paper,  felt,  charcoal,  etc.  The 
pores  of  these  substances  are  sufficiently  large  to  allow  liquids  to 
pass,  but  small  enough  to  arrest  the  particles  held  in  suspension. 
Figure  2  represents  a  filtering  fountain,  one  side  of  which  is  sup- 


Fig.  2. 


Fig.  3- 


posed  to  have  been  removed,  so  that  its  construction  can  be  seen.  It 
consists  of  a  box  about  a  yard  high  divided  in  the  inside  into  two 
compartments  by  a  porous  slab,  A.  The  water  to  be  filtered  is 
placed  in  the  upper  compartment,  whence  it  slowly  percolates 
through  the  pores  of  the  stone  into  the  lower  one,  leaving  behind 
it  the  foreign  substances.  In  one  of  the  sides  of  the  box  is  a  tube 
a,  which  terminates  in  the  lower  compartment,  and  allows  the  air 
to  escape  in  proportion  as  water  enters. 

Figure  3  represents  a  filter  known  as  the  strainer  of  Hippocrates. 
It  is  a  conical  felt  bag  suspended  by  three  cords,  into  which  is 
poured  the  turbid  liquor ;  it  slowly  traverses  the  pores,  while  all 


-11] 


Compressibility. 


the  solid  particles  to  which  the  turbidity  is  due,  remain  behind  on 
the  filter.  This  method  is  well-adapted  for  clarifying  syrups,  jel- 
lies, and  liqueurs. 

Layers  of  powdered  wood  charcoal  are  also  used  for  filtration. 
A  layer  of  sand  or  of  broken  glass  produces  the  same  effect.  The 
limpidity  of  well-water  is  due  to  the  filtration  through  strata  of  earth. 

1 1 .  Compressibility. — This  is  the  property  which  bodies  possess 
of  being  diminished  in  volume  by  pressure  without  undergoing  any 
loss  of  weight.  Being  due  to  the  approach  of  the  molecules,  it  is 
both  a  consequence  and  a  proof  of  porosity. 

Compressibility  is  very  marked  in  sponge,  caoutchouc,  cork, 
pith,  paper,  cloth,  etc.  Their  volume  is  considerably  diminished 
by  mere  pressure  between 
the  fingers.  The  compres- 
sibility of  metals  is  proved 
by  the  impression  which  they 
receive  from  the  die,  in  the 
process  of  coining.  There 
is,  in  most  cases,  a.  limit 
beyond  which,  when  the 
pressure  is  increased,  solids 
are  fractured  or  reduced  to 
powcjer. 

The  compressibility  of 
liquids  is  so  small  as  to  have 
remained  for  a  long  time  un- 
detected :  it  may,  however, 
be  proved  by  experiment,  as 
will  be  seen  in  the  chapter 
on  HYDROSTATICS  (74). 

The  most  compressible 
bodies  are  gases,  which  by 
pressure  may  be  made  to 
occupy  ten,  twenty,  or  a 
hundred  times  less  space 
than  under  ordinary  circum- 
stances. The  great  com- 
pressibility of  gases  may  be 


Fig.  4. 


demonstrated  by  means  of  a  glass  tube  with  very  thick  sides, 
closed  at  one  end  and  provided  with  a  tight-fitting  solid  piston 
(fig.  4).  The  enclosed  air  cannot  escape,  and  yet  when  the  handle 


io     Properties  of  Matter  and  Universal  Attraction.    [11- 

of  the  piston  is  pressed  it  can  be  moved  down  to  one-half  to 
three-quarters  the  length  of  the  tube  ;  proving  that  the  volume  of 
the  air  is  reduced  necessarily  to  half  or  a  quarter  what  it  was  origi- 
nally. Most  gases,  when  thus  compressed,  exhibit  a  remarkable 
property,  to  which  we  shall  afterwards  return,  that,  namely,  of 
liquefying  or  passing  from  the  gaseous  to  the  liquid  state. 

12.  Elasticity. — Elasticity  is  the  property  which  bodies  possess 
of  resuming  their  original  form  or  volume,  when,  after  having  been 
compressed,  bent,  twisted,  or  pulled,  the  force  which  altered  them 
has  ceased  to  act. 

Four  kinds  of  elasticity  may  be  distinguished  :  the  elasticity  by 
pressure,  as  in  the  case  of  gases  ;  the  elasticity  by  flexure  or  bending, 
observed  in  springs  ;  the  elasticity  of  torsion  or  twisting,  which  is 
produced  in  linen  or  cotton  threads  when  they  are  untwisted  ;  and, 
finally,  the  elasticity  of  tension  or  stretching,  which  is  that  of  piano 
or  violin  strings  when  they  are  stretched. 

Whatever  be  the  kind  of  elasticity,  it  is  always  due  to  a  displace- 
ment of  molecules.  If  the  molecules  have  been  brought  nearer  by- 
pressure,  the  repulsive  force  of  heat  tends  to  separate  them  ;  if,  on 
the  contrary,  they  have  been  separated,  molecular  attraction  tends 
to  bring  them  near  each  other  again.  If  a  piece  of  whalebone 
be  bent,  the  molecules  in  the  concave  part  being  compressed  repel 
each  other ;  in  the  convex  part,  where  they  are  separated,  they 
tend  to  approach  each  other  ;  both  these  actions  tend,  therefore, 
to  straighten  it  as  soon  as  it  is  free. 

Gases  and  liquids  are  perfectly  elastic;  in  other  words,  they 
regain  exactly  the  same  volume  when  the  pressure  becomes  the 
same.  Solid  bodies  present  different  degrees  of  elasticity,  though 
none  present  the  property  in  the  same  perfection  as  liquids  and 
gases,  and  in  all  it  varies  according  to  the  time  during  which  the 
body  has  been  exposed  to  pressure.  Caoutchouc,  ivory,  glass, 
and  marble  possess  considerable  elasticity;  lead,  clay,  and  fats 
scarcely  any. 

There  is  a  limit  to  the  elasticity  of  solids,  beyond  which  they 
either  break  or  are  incapable  of  regaining  their  original  form  and 
volume.  In  sprains,  for  instance,  the  elasticity  of  the  tendons  has 
been  exceeded,  so  also  when  glass  breaks.  In  gases  and  liquids, 
on  the  contrary,  no  such  limit  can  be  reached  ;  they  always  regain 
their  original  volume. 

The  elasticity  of  solids  may  be  demonstrated  by  the  following 
experiment : — On  a  slab  of  polished  black  marble  thinly  smeared 


-13] 


Applications  of  Elasticity. 


II 


with  oil,  an  ivory  ball  is  allowed  to  drop  from  gradually  increasing 
heights.  Each  time  it  will  rebound  and  rise  to  a  height  a  little  less 
than  that  from  which  it  fell, 
after  having  formed  on  the  layer 
of  oil  a  circular  impression 
which  is  larger  the  greater  the 
height  of  the  fall  (fig.  5).  From 
this  we  conclude  that  the  ball 
was  flattened  each  time,  and 
that  it  rebounded  in  conse- 
quence of  the  reaction  of  its 
compressed  molecules. 

13.  Applications  of  elas- 
ticity.— Numerous  applications 
of  the  property  of  elasticity 
may  be  mentioned.  It  is  owing 
to  their  elasticity  that  corks 


Fig.  5- 


are  used  for  closing  bottles. 
Pushed  into  the  neck  by  the 
exercise  of  a  certain  force,  they 
become  compressed,  and  then 
their  elasticity  causing  them  to 
press  against  the  sides,  they  completely  close  the  neck. 

Children's  balls  depend  upon  the  elasticity  of  gas  :  they  are  made 
of  caoutchouc,  and  are  inflated  by  air  ;  when  they  strike  against  the 
ground,  or  against  a  wall,  their  volume  diminishes,  and  the  air  which 
they  contain  being  suddenly  compressed,  expands,  and,  acting  like 
a  spring,  makes  the  ball  rebound.  A  similar  application  is  met 
with  in  air-cushions.  They  are  made  of  an  air-tight  niaterial,  and 
being  inflated  by  air,  are  both  compressible  and  elastic,  and  thus 
form  a  very  soft  seat. 

Ah -guns  are  a  further  application.  The  breech  in  these  is  made 
of  steel,  and  is  hollow  ;  air  is  compressed  in  it  by  means  of  an  in- 
strument called  the  compression-pump,  and  being  suddenly  liberated 
its  expansive  force  is  sufficient  to  expel  the  projectile. 

The  use  of  carriage,  and  of  watch  and  clock,  springs  depends 
upon  the  elasticity  of  steel.  In  like  manner  the  elasticity  of  wool, 
hair,  feathers,  is  made  use  of  in  mattresses,  pillows,  and  seats. 

Lastly,  it  is  owing  to  their  elasticity  that  piano,  guitar,  or  violin 
strings  are  capable  of  being  put  into  a  vibratory  motion,  which, 
as  we  shall  prove,  is  the  origin  of  the  sounds  which  stringed  instru- 
ments yield. 


1 2     Properties  of  Matter  and  Universal  A  ttr action.    [14- 


CHAPTER   III. 

MOTION   AND    FORCE. 

14.  Rest  and  motion. — To  understand  what  we  have  to  say 
about  inertia,  weight,  universal  gravitation,  and  the  motion  of 
liquids  and  gases,  it  is  first  of  all  necessary  to  give  some  very 
elementary  notions  about  motion  and  force. 

A  body  is  said  to  be  at  rest,  when  it  remains  in  the  same  place  ; 
to  be  in  motion  when  it  passes  from  one  place  to  another.  Both 
rest  and  motion  are  either  absolute  or  relative. 

Absolute  rest  would  be  the  entire  absence  of  motion.  No  such 
condition,  however,  is  known  in  the  universe  ;  for  the  earth  and  the 
other  planets  rotate  both  about  the  sun  and  about  their  own  axes  ; 
and  therefore,  all  the  parts  composing  them  share  this  double 
motion.  Even  the  sun  itself  has  a  motion  of  rotation  which  ex- 
cludes the  idea  of  absolute  rest. 

Relative  or  apparent  rest  is  the  condition  of  a  body  which 
appears  fixed  in  reference  to  surrounding  objects,  but  which  really 
shares  with  them  a  double  motion.  For  instance,  a  passenger  in  a 
railway  carriage  may  be  in  a  slate  of  relative  rest  with  respect  to 
the  train  in  which  he  travels,  but  he  is  in  a  state  of  relative  motion 
with  respect  to  the  objects  (fields,  houses,  etc.)  past  which  the  train 
rushes.  These  houses  again  enjoy  merely  a  state  of  relative  rest, 
for  the  earth  itself  which  bears  them  is  in  a  state  of  incessant  rela- 
tive motion  with  respect  to  the  celestial  bodies  of  our  solar  system. 

The  absolute  motion  of  this  passenger  would  be  that  measured 
in  regard  to  a  fixed  point  in  space,  which  cannot  be  realised,  for  we 
know  no  such  point.  In  short,  absolute  motion  and  rest  are  un- 
known to  us  ;  in  nature,  relative  motion  and  rest  are  alone  presented 
to  our  observation. 

15.  Different  kinds  of  motion. — Motion  is  either  rectilinear  or 
curvilinear :  rectilinear  when  the  moving  body  travels  along  a 
straight  line,  as  when  a  body  falls  to  the  ground  ;  curvilinear  when 
it  goes  along  a  curved  line,  as  in  the  case  of  a  horse  turning  in  a 
mill. 

Each  kind  of  motion  is  either  uniform  or  varied. 


-18]  Inertia.  i  3 

1 6.  uniform  motion. — Motion  is  said  to  be  uniform  when  the 
moving  body  passes  over  equal  spaces  in  equal  intervals  of  time  ; 
such,  for  instance,  as  the  motion  of  a  water-wheel  when  it  makes 
exactly  the  same  number  of  turns  in  a  minute.     Such,  again,  is  the 
motion  of  the  hands  of  a  watch.     A  regiment  of  soldiers  marching 
in  step  affords  a  further  example  of  uniform  motion. 

The  velocity  of  motion  is  the  space  traversed  in  a  given  time,  a 
second  or  an  hour,  for  example.  A  train  which  moves  thirty  miles 
in  each  successive  hour  is  said  to  have  a  velocity  of  thirty  miles  an 
hour. 

17.  Varied  motion. — Varied  motion  is  that  in  which  unequal 
spaces  are  traversed  in  equal  times.     If  the  spaces  traversed  in  the 
same  time  go  on  increasing,  the  motion  is  said  to  be  accelerated ; 
such  is  the  motion  of  a  train  starting  from  a  station  ;  if  the  spaces 
decrease,  as  is  the  case  when  a  train  comes    into  a  station,  the 
motion  is  retarded. 

If  the  distances,  traversed  in  equal  times,  always  increase  by  the 
same  amount,  the  motion  is  said  to  be  uniformly  accelerated ;  if, 
on  the  other  hand,  they  constantly  decrease  by  the  same  amount, 
the  motion  is  uniformly  retarded.  We  shall  soon  see  examples  of 
these  kinds  of  motion  in  the  case  of  falling  bodies. 

1 8.  Inertia. — Inertia  is  a  purely  negative  property  of  matter  ; 
it  is  the  incapability  of  matter  to  change  its  own  state  of  motion  or 
of  rest. 

Daily  observation  shows  that  a  body  never  spontaneously  passes 
from  a  state  of  rest  into  one  of  motion.  Bodies  in  falling  to  the 
ground  seem  to  set  themselves  in  motion.  This  is,  however,  not  in 
consequence  of  any  inherent  property  ;  but,  as  we  shall  afterwards 
see,  because  they  are  acted  upon  by  the  force  of  gravity. 

Not  merely  do  bodies  at  rest  persist  in  a  state  of  rest,  but  bodies 
in  motion  continue  to  move.  This  principle  may  seem  less  obvious 
than  the  former,  because  we  are  accustomed  to  see  many  bodies 
gradually  move  more  slowly,  and  ultimately  stop,  as  is  the  case 
with  a  billiard-ball,  for  example.  But  this  is  not  due  to  any  inhe- 
rent preference  for  a  state  of  rest  on  the  part  of  the  billiard-ball,  but 
because  its  motion  is  impeded  by  the  friction  of  the  cloth  on  which 
it  rolls,  and  by  the  resistance  of  the  air.  The  smaller  these  resist- 
ances, the  more  prolonged  is  its  motion  ;  as  is  observed,  for 
instance,  if  a  ball  be  set  rolling  on  a  smooth  sheet  of  ice.  If  all  im- 
p'eding  causes  were  removed,  a  body  once  in  motion  would  con- 
tinue to  move  for  ever. 


14    Properties  of  Matter  and  Universal  Attraction.    [19- 

19.  Effects  due  to  inertia. — Innumerable  phenomena  may  be 
explained  by  the  inertia  of  matter.     For  instance,  before  leaping 
a  ditch  we  run  towards  it,  in  order  that  the  motion  of  our  bodies 
at  the  time  of  leaping  may  add  itself  to  the  muscular  effort  then 
made. 

On  descending  carelessly  from  a  carriage  in  motion,  the  upper 
part  of  the  body  retains  its  motion,  whilst  the  feet  are  prevented 
from  doing  so  by  friction  against  the  ground  ;  the  consequence  is 
we  fall  towards  the  moving  carriage. 

If  a  man  in  running  strikes  his  foot  against  an  obstacle  he  is  apt 
to  fall  down  in  front,  because  the  rest  of  his  body  tends  to  retain 
the  motion  it  has  acquired.  When  a  horse  at  full  gallop  suddenly 
stops,  if  the  rider  does  not  hold  fast  with  his  knees,  he  is  thrown 
over  the  horse's  head  in  virtue  of  his  inertia.  A  grindstone  only 
gradually  acquires  its  full  speed,  but  then  continues  its  movement 
even  after  the  force  has  ceased  to  act. 

The  terrible  accidents  on  our  railways  are  chiefly  due  to  inertia. 
When  the  motion  of  the  engine  is  suddenly  arrested  the  carriages 
strive  to  continue  the  motion  they  had  acquired,  and  in  doing  so 
are  shattered  against  each  other. 

The  action  of  projectiles  is  another  case.  W7hen  a  bullet 
traverses  a  wall,  or  cuts  a  tree  in  two,  it  is  owing  to  its  tendency  to/ 
retain  the  velocity  which  the  explosion  of  the  powder  had  imparted 
to  it.  In  the  action  of  hammers,  and  of  pile  driving,  we  have 
analogous  cases. 

The  actions  of  beating  a  coating  with  a  stick  to  expel  dust ;  of 
shaking  the  snow  from  our  shoes  by  kicking  against  the  door-post ; 
of  cleaning  a  dusty  book  by  striking  it  against  another,  all  depend 
upon  the  property  of  inertia.  The  hoop,  the  top,  and  other  toys  are 
further  illustrations. 

20.  Forces,  powers,  resistances. — Bodies  being  of  themselves 
inert,  and  having  no  tendency  to  change  either  their  state  of  rest  or 
that  of  motion,  any  cause  capable  of  making  them  pass  from  a  state 
of  rest  to  one  of  motion,  or  conversely  from  a  state  of  motion  to 
one  of  rest,  is  called  a  force. 

The  attractions  and  repulsions  exerted  between  the  rnolecules 
are  forces  ;  the  muscular  action  which  men  and  animals  bring  into 
play  is  a  force,  as  is  also  the  elasticity  of  gases  and  vapours  which 
we  shall  subsequently  discuss. 

The  forces  which  tend  to  produce  motion  are  called  powers; 
these  which  tend  to  destroy  motion  are  called  resistances.  Thus, 


-22]  Friction.  1 5 

when  a  man  drags  a  burden  along  the  ground  his  muscular  force  is 
a  power,  while  the  friction  of  the  burden  against  the  ground  is  a 
resistance. 

Forces  of  the  kind  called  powers  are  always  tending  to  accelerate 
motion,  and  are  called  accelerating  forces.  Resistances,  on  the 
contrary,  always  tending  to  retard  it,  are  called  retarding  forces. 

21.  Friction.  —  The    surfaces    of    bodies    are   never    perfectly 
smooth  ;  even  the  smoothest  possess  roughnesses  which  cannot  be 
detected  by  the  touch  nor  by  ordinary  sight ;  and  friction  consists 
in  the  fact  that  the  body  must  be  raised  over  these  obstacles  or 
must   break   them   down.     They   fit   in   each   other  like  toothed 
wheels. 

Friction  is  of  two  kinds  :  sliding,  in  which  one  body  slides  over 
another,  as  when  a  box  is  dragged  along  a  floor  ;  it  is  least  when 
the  two  surfaces  are  always  in  contact,  as  in  the  motion  of  an  axle 
in  its  bearing  ;  and  rolling  friction,  as  when  a  cylindrical  body 
moves  over  a  horizontal  surface  like  an  ordinary  wheel. 

Friction  is  lessened  by  rubbing  on  the  surfaces  in  contact  fatty 
materials  which  are  not  absorbed  by  them.  Moisture  and  oil  in- 
crease the  friction  of  wood,  for  they  are  absorbed  by  it,  while 
tallow,  soap,  and  black  lead  lessen  it.  Oil  and  lard  lessen  the 
friction  of  metallic  surfaces.  Rolling  friction  is  less  than  sliding 
friction,  hence  the  use  of  castors  on  pianos  and  other  heavy  furni- 
ture. On  the  other  hand,  rolling  is  sometimes  changed  into  sliding 
friction,  in  order  to  increase  it,  as  when  a  drag  is  applied  to  awheel. 
The  friction  of  carriage-wheels  is  less,  the  greater  the  diameter  of 
the  wheel  and  the  less  that  of  the  axle. 

Without  friction  on  the  ground  neither  man  nor  animals, 
neither  ordinary  carriages  nor  railway  ones,  could  move  ;  without 
it  no  book  would  remain  on  a  desk,  and  without  it  we  could  hold 
nothing  in  the  hands. 

22.  Distinctive  characters  of  forces. — Three  things  are  to  be 
distinguished  in  each  force  ;  the  point  of  application,  the  direction, 
and  the  intensity. 

T\*&  Point  of  application  of  a  force  is  the  point  at  which  it  exerts 
its  action.  Having  attached  a  cord  to  a  sledge,  as  shown  in  fig. 
6,  the  point  of  application  is  the  point  A,  at  which  the  cord  is 
actually  attached. 

The  direction  of  a  force  is  the  right  line  along  which  it  urges"  or 
tends  to  urge  the  point  of  application.  In  rig.  6  the  cord  A  B  re- 
presents the  direction  of  the  force. 


1 6    Properties  of  Matter  and  Universal  A  ttraction.    [22- 

The  intensity  of  a  force  is  its  energy,  its  magnitude,  or  value, 
in  reference  to  a  certain  standard.  In  fig.  6,  which  represents  a 
horse  drawing  a  cask  on  a  sledge,  a  certain  exertion  of  force  is 


Fig.  6. 

required  on  the  part  of  the  horse  ;  if  the  sledge  were  loaded  twice 
or  thrice  as  much,  the  force  required  must  be  twice  or  thrice  as 
great. 

23.  Measurement  of  forces.  Dynamometer. — The  force  which 
a  motor  (32)  developes  in  pushing  or  drawing  a  body,  is  measured 
by  the  number  of  pounds  necessary  to  produce  the  same  pressure  or 
the  same  pull ;  so  that  a  force  is  said  to  be  a  force  of  40  or  50 
pounds,  when  it  can  be  replaced  by  the  action  of  a  weight  of  4.0  or 
50  pounds. 

The  weight  which  thus  represents  the  intensity  of  a  force  is 
determined  by  means  of  the  dynamometer.  There  are  several 
forms  of  this  instrument,  one  of  the  simplest  being  that  represented 
in  fig.  7.  It  consists  of  a  V-shaped  plate  of  tempered  steel,  AB.  At 
one  end  of  the  arm  B  is  fixed  an  iron  arc,  ;/,  which  passes  freely 
through  an  aperture  at  the  end  of  the  arm  A.  To  this  latter  is 
fixed  an  arc,  ;«,  fitting  in  the  same  manner  in  the  arm  B.  The  arc 
m  is  provided  at  the  end  with  a  crook,  and  n  with  a  ring,  and  on 
the  latter  n  there  is  a  graduation  obtained  in  the  following 
manner  : — 

The  apparatus  being  fixed  to  a  resisting  support,  weights  of  i,  2, 
3,  4,  or  more  pounds  are  successively  suspended  to  the  crook.  The 
arm  B,  supported  by  the  arc  «,  remains  fixed,  while  the  arm  A, 
being  moved  by  the  weight  attached  to  the  arc  ;;/,  is  lowered  to  an 
extent  dependent  on  the  weight.  The  load,  is  gradually  increased 
until  it  has  readied  the  utmost  limit  possible  without  breaking,  care 


-24] 


Resultant  and  Component  Forces. 


being  taken  at  each  load  to  mark  a  line  on  the  arc  n  at  the  point 
at  which  the  arm  A  stops. 


Fig-  7. 


Fig. 


In  order  to  apply  it  to  the  measurement  of  forces,  to  estimate, 
for  instance,  the  effort  necessary  to  drag  a  load  (fig.  8),  the  crook  of 
the  arc  m  is  fixed  to  the  load,  then  holding  in  the  hand  the  ring  of 
the  arc  n  it  is  pulled  until  the  load  is  moved.  The  flexure  of  the 
arm  A  marks  on  the  arc  n  the  value  in  pounds  of  the  effort  of 
traction. 

The  apparatus  described  is  also  used  as  a  balance  to  determine 
the  weight  of  bodies,  and  is  known  as  the  steelyard. 

When  forces  are  once  measured  or  expressed  in  weight,  they 
may  be  represented  as  to  their  intensity  by  means  of  the  line  which 
indicates  their  direction.  For  this  purpose  a  length  is  measured  off 
on  this  line,  starting  from  the  point  of  application,  which  contains 
the  unit  of  length  as  many  times  as  the  intensity  of  the  force  con- 
tains pounds.  Thus,  if  in  fig.  6  the  effort  of  traction  is  1 5  pounds, 
a  length,  AB,  would  be  measured  from  A  equal  to  1 5  times  the  unit 
of  length,  which  may  be  an  inch  for  distance.  Thus  the  work  of 
the  horse  in  drawing  the  sledge  would  be  represented  both  in 
direction  and  intensity  by  the  line  AB. 

24.  Resultant  and  component  forces. — When  a  body  is  acted 
upon  by  only  a  single  force,  it  is  clear  that,  if  it  is  not  hindered  by 
any  obstacle,  it  will  move  in  the  direction  of  this  force  ;  but  if  it  is 
simultaneously  acted  upon  by  several  forces  in  different  directions, 
its  direction  will  not,  speaking  generally,  coincide  with  that  of  any 

C 


1 8    Properties  of  Matter  and  Universal  Attraction.  [24- 

one  of  these  forces.    If  two  men,  for  example,  on  the  banks  of  a  river, 
tow  a  boat  by  means  of  ropes,  as  shown  in  fig.  9,  the  boat  follows 


Fig.  9. 

neither  the  direction  AB,  nor  the  direction  AC,  in  which  these  men 
are  respectively  pulling,  but  takes  an  intermediate  direction,  AE  ; 
that  is,  it  moves  as  if  it  were  acted  upon  by  a  single  force  in  the 
direction  AE. 

As  the  single  force,  which  we  conceive  as  having  the  direction 
AE,  produces  the  same  effect  as  the  forces  of  traction  of  these  two 
men,  is  called  the  resultant  of  these  two  forces ;  and  conversely  these, 
in  reference  to  their  resultant,  are  spoken  of  as  the  components. 

2$.  Value  of  the  resultant  of  two  concurring:  forces.  Paral- 
lelogram of  forces. — When  two  forces  having  different  directions 

are  applied  to  the  same  point 

A^ .B  of    a  body,   as    represented 

in  fig.  9,  there  is  a  very 
simple  ratio  between  their 
intensities  and  the  intensity 
of  their  resultant,  which  is 
of  great  importance  from  the 
number  of  its  applications. 
It  will  first  of  all  be  necessary  to  define  the  word  parallelogram, 
of  which  we  shall  make  use.  The  parallelogram  is  a  geometrical 
figure,  formed  of  four  right  lines,  each  pair  of  which  is  parallel  (fig. 
10),  that  is,  the  two  lines  AB  and  CD  are  parallel,  and  also  the 
lines  AD  and  BC  These  lines  form  the  sides  of  the  parallelogram, 
and  the  po  nts  A,  B,  C,  D,  the  angles.  The  diagonal  is  the  line, 
like  AC,  joining  two  opposite  angles  A  and  C. 

In  treatises  on  mechanics,  proofs  are  given  of  the  following  im- 


Fig.  10. 


-26]  Parallelogram  of  Forces.  19 

portant  theorem,  which  is  known  as  the  principle  of  the  parallelo- 
gram of  forces  : 

When  two  forces  applied  at  the  same  point  A  (fig.  1 1)  are  repre- 
sented in  direction,  and  in  intensity  by  the  sides  AB  and  AD  of  the 


Fig.  IT. 

parallelogram  ABCD,  their  resultant  is  represented  both  as  to  its 
intensity  and  direction  by  the  diagonal  AC  of  this  parallelogram. 

That  is,  that  the  point  A  being  simultaneously  acted  upon  by  two 
forces,  whose  directions  and  intensities  are  respectively  represented 
by  AB  and  AD  ;  this  point  moves  in  the  direction  AC  exactly  as 
if  it  were  acted  upon  by  a  single  force,  the  direction  and  intensity 
of  which  are  represented  by  the  line  AC. 

Frequent  applications  are  met  with  of  the  principle  of  the  paral- 
lelogram of  forces.  Thus,  in  the  flight  of  a  bird,  when  the  wings 
strike  against  the  air,  a  resistance  is  offered  which  is  equal  to  an 
impulsive  force  from  back  to  front  in  the  directions  AH  and  AK 
(fig.  12) ;  hence,  representing  by  AB  and  AD,  the  intensities  and  di- 
rections of  these  impulsive  forces,  if  the  parallelogram  be  completed, 
we  shall  find  that  the  resultant,  or  the  single  force  which  makes  the 
bird  advance,  is  represented  in  direction  and  magnitude  by  the 
diagonal  AC.  The  same  reasoning  applies  to  the  swimming  both 
of  men  and  fishes. 

26.  Another  effect  of  tbe  parallelogram  of  forces. — We  have 
seen  that,  in  accordance  with  the  principle  of  the  parallelogram  of 


2O    Properties  of  Matter  and  Universal  Attraction.  [26- 

forces,  two  forces  applied  at  the  same  point  of  a  body  may  be  re- 
duced to  a  single  one.     By  the  aid  of  the  same  principle  a  single 


Fig.  12 

force  applied  to  a  body  may  be  replaced  by  two  other  forces  pro- 
ducing together  the  same  effect  as  the  first.  This  force  is  then  said 
to  be  decomposed  into  two  others. 

It  is  but  seldom  indeed  that  the  action  of  a  force  is  entirely 
utilised ;  it  may  almost  always  be  decomposed  into  two  others,  only 
one  of  which  produces  a  useful  effect.  Thus  when  the  wind  blows 


Fig.  13. 

against  the  sails  ot  a  vessel,  not  quite  directly,  but  a  little  on  one 
side,  as  shewn  in  fig.  13,  the  effect  of  the  wind  in  the  direction  va 


-28]  Equilibrium  of  Forces.  .        21 

may  be  considered  to  be  resolved  into  two  others,  one  in  the  direc- 
tion ca,  and  the  other  in  a  lateral  direction  ba,  of  which  the  first 
moves  the  vessel.  The  second  only  guides  it. 

27.  Case   in  which   the   forces  are  parallel.     Value  of  the 
resultant. — In  the  case  of  the  boat  drawn  by  a  rope  (fig.  9),  the 
forces  were  concurrent,  that  is,  their  directions  if  produced  would 
meet  in  one  point ;  but  it  may  happen  that  the  forces  applied  to  the 
same  body  are  parallel,  and  then  two  cases  present  themselves  ; 
that  is,  they  either  act  in  the  same  direction  as  in  the  case  of  two 
horses  drawing  a  carriage  ;  or  they  may  act  in  opposite  directions  ; 
when  a  steamer  for  instance  ascends  a  river,  the  current  acts  in 
.opposition  to  the  force  which  urges  the  steamer.     It  can  be  proved 
that,  in  the  first  case,  the  resultant  of  the  forces  is  equal  to  their 
sum  ;  and  that  in  the  second  it  is  equal  to  their  difference. 

28.  Equilibrium  of  forces. — When  several  forces  act  upon  a 
body  at  the  same  time,  they  do  not  always  put  it  in  motion;  it  may 
happen  that  while  some  of  these  forces  tend  to  produce  motion  in  a 
certain  direction,  the  others  tend  to  produce  an  equal  and  contrary 
motion  in  the  opposite  direction.     It  is  clear  that  in  this  case,  since 
the  forces  just  neutralise  each  other,  no  effect  can  be  produced. 
Whenever  several  forces  applied  to  the  same  body  thus  mutually 
destroy  each  other,  we  have  what  is  called  equilibrium. 

The  simplest  case  of  equilibrium  is  that  of  two  equal  and  oppo- 
site forces  applied  at  the  same  point  of  a  body.  For  instance,  if 
two  men  pull  at  a  cord  with  the  same  intensity,  one  in  one  direc- 
tion, and  the  other  in  the  opposite  One,  equilibrium  will  be  produced 
(fig.  14).  In  like  manner  if,  in  a  well,  two  buckets  of  the  same  size 


Fig.  14. 

each  full  of  water,  are  suspended  at  the  end  of  a  rope  which  passes 
round  a  pulley,  the  weight  of  one  holds  the  other  in  equilibrium. 

The  bodies  which  we  consider  ordinarily  to  be  in  a  state  of  rest, 
are  really  in  a  state  of  equilibrium.  For  instance,  when  a  body 
rests  on  a  table,  there  is  equilibrium  between  the  force  of  gravity 
which  tends  to  make  the  body  fall,  and  the  resistance  which  the 
table  offers  to  the  fall.  If  the  weight  of  the  body  exceeds  this  re- 


22    Properties  of  Matter  and  Universal  Attraction.  [28- 

sistance,  equilibrium  is  destroyed,  the  table  is  broken,  and  the  body 
falls. 

29.  Centrifugal  force. — We  shall  conclude  these  notions  about 
forces  by  mentioning  a  force  to  which  curvilinear  motion  is  due, 
namely  centrifugal  Jorce.  This  may  be  explained  as  follows. 
Whenever  a  body  has  been  put  in  motion  in  a  particular  direction, 
in  virtue  of  its  inertia,  it  tends  always  to  move  in  this  direction. 
Hence  whenever  a  line  is  seen  to  move  in  a  circle,  this  can  only  be 
due  to  some  obstacle,  or  some  new  force  which  deviates  it.  b 
fact,  since  a  curved  line  may  be  considered  to  consist  of  a  series  of 
infinitely  small  straight  lines,  the  moving  body,  owing  to  its  inertia, 
always  tends  to  follow  the  prolongation  of  the  small  straight  line 
which  it  traverses.  It  tends  then  to  retain  its  motion  in  a  straight 
line,  and  to  fly  from  the  curve  which  it  is  compelled  to  describe. 
This  action  is  called  the  centrifugal  force,  from  two  Latin  words 
which  signify  to  fly  from  the  centre. 


The  production  of  centrifugal  force  in  circular  motion  may  be  de- 
monstrated by  means  of  the  apparatus  represented  in  fig.  15.     On 


-31]  Flattening  of  the  Earth  at  the  Poles.  23 

a  brass  frame  AB  is  stretched  a  stout  brass  wire,  and  on  which  are 
slid  two  ivory  balls  which  can  move  freely  along  the  wire  :  the  balls 
being  arranged  as  shown  in  the  figure,  the  frame  is  rapidly  rotated 
by  means  of  the  turning  table.  The  balls,  projected  by  the  centri- 
fugal force,  glide  along  the  wire  ;  and  strike  the  ends  with  the 
greater  force,  the  greater  the  velocity  of  rotation. 

30.  Effects    of  centrifugal   force. — The   centrifugal   force    is 
greater  the  greater  the  velocity,  and  the  more  marked  the  curvature 
of  the  line  along  which  the  movable  body  passes.     Hence  railways 
should  be  as  straight  as  possible,  for  as  the  trains  have  a  great 
velocity,  when  they  move  along  a  curve  the  centrifugal  force  is  con- 
tinually tending  to  throw  them  off,  and  the  more  so  the  sharper  the 
curve. 

It  is  owing  to  centrifugal  force  that  the  wheels  of  a  carriage 
moving  along  a  muddy  road  throw  off  the  mud  that  adheres  to  the 
rim. 

In  a  circus,  the  horses  and  their  riders  always  incline  their  bodies 
towards  the  centre,  and  the  greater  their  speed  the  greater  their  in- 
clination. The  object  of  this  is  to  allow  their  weight  to  counteract 
the  influence  of  the  centrifugal  force,  which  would  throw  them  off 
if  they  stood  upright. 

In  sugar  refineries  centrifugal  force  is  applied  in  removing  syrup 
from  crystallised  sugar.  The  sugar  is  placed  in  a  cylindrical  ves- 
sel, whose  sides  are  made  of  wire  gauze,  and  which  is  put  in  rapid 
rotation.  The  centrifugal  force  scatters  the  coloured  syrup  through 
the  meshes  of  the  sieve,  while  the  solid  crystals  are  left  behind 
colourless  and  pure.  The  same  principle  is  applied  in  drying 
clothes  in  large  washing  establishments.  A  wet  mop  made  to  turn 
quickly  about  its  own  handle  as  an  axis  throws  the  water  off  on  all 
sides,  and  quickly  dries  itself. 

A  hoop  trundled  along  the  ground  may  move  for  a  long  time  be- 
fore falling,  but  if  we  attempt  to  keep  it  upright  while  in  a  state  of 
rest,  it  at  once  falls.  The  reason  of  this  is  that,  while  in  motion,  if 
it  inclines  to  one  side,  the  inclination  causes  it  to  describe  a  curved 
line,  whence  arises  a  centrifugal  force  which  opposes  the  fall  of  the 
hoop  at  any  rate  so  long  as  it  retains  a  sufficient  velocity. 

31.  Flattening:  of  the  earth  at  the  poles.— One  of  the  most 
remarkable  effects  of  centrifugal  force  is  the  flattening  of  the  earth 
at  the  two  poles.     To  explain  this  phenomenon  we  must  premise 
that  the  earth,  which  is  nearly  spherical  in  form,  rotates  about  an 
imaginary  axis  passing  through  its  two  poles,  and  that,  in  this  ro- 
tation, all  points  on  the  surface  have  not  the  same  velocity,  for  they 


24    Properties  of  Matter  and  Universal  Attraction.  [31 


do  not  describe  the  same  paths.  For,  at  the  equator,  they  describe 
every  twenty-four  hours  a  circumference  equal  to  that  of  the  earth, 
while  points  taken  at  increasing  distances  from  the  equator  gradu- 
ally describe  smaller  and  smaller  circles  to  the  poles  where  they 
have  no  motion.  Hence,  owing  to  the  daily  rotation  about  the 
earth's  axis,  a  centrifugal  force  is  produced  which  is  greatest  at  the 
equator,  and  gradually  diminishes  up  to  the  poles,  where  there  is 
none  at  all.  Hence,  owing  to  this  inequality  in  the  intensity  of  the 
centrifugal  force,  there  must  arise  an  accumulation  of  matter  about 
the  equator,  especially  if,  as  geologists  assume,  the  earth  was  origi- 
nally in  a  state  of  fusion. 

It  has  in  fact  been  ascertained  by  di- 
rect measurement,  that  the  radius  of  the 
earth  at  the  poles  is  less  than  that  at  the 
equator  by  about  —9  the  latter,  or  13^ 
miles.  A  similar  flattening  has  been  ob- 
served in  other  planets. 

To  demonstrate  this  bulging  at  the 
equator  and  flattening  at  the  poles,  use  is 
made  of  the  apparatus  represented  in 
fig.  1 6.  It  consists  of  an  iron  rod,  which 
may  be  fixed  upon  the  turning-table,  in- 
stead of  the  piece  A  B  (fig.  1 5).  At  the 
bottom  of  the  rod  are  fixed  four  thin 
elastic  metal  plates,  which  are  joined  at 

the  top  to  a  ring  which  can  slide  up  and  down  the  rod.  The 
apparatus  being  then  put  in  rapid  rotation,  the  rings  slide  down 
the  rod  as  represented  in  the  figure  to  an  extent  depending  on  the 
rapidity  of  the  rotation. 


LEVERS.  •  +> 

32.  Mechanics.  Machines. — Mechanics  is  the  science  which 
treats  of  forces  and  of  motion.  Several  forces  being  applied  to  the 
same  body,  it  indicates  the  relation  which  must  exist  between  them 
in  order  to  produce  equilibrium,  or  in  order  to  produce  a  given 
effect. 

Any  apparatus  which  serves  to  transmit  the  action  of  a  force  is 
a  machine  ;  and  any  force  which  moves  a  machine  is  a  motor.  In 
cutting  an  apple  with  a  knife,  the  hand  is  the  motor,  and  the  knife 
which  transmits  its  action  is  a  machine.  A  horse  drawing  a  cart  is 


-33]  Levers.  25. 

a  motor,  and  the  cart  which  utilises  the  force  of  the  horse  in  con- 
veying loads  is  a  machine.  The  watercourse  which  works  a  wheel, 
the  wind  which  turns  a  mill,  and  the  steam  which  moves  a  locomo- 
tive, are  all  motors  ;  and  the  water-wheel,  the  wind-mill,  and  the 
locomotive  are  all  machines. 

Machines  do  not  increase  the  force  of  a  motor:  whatever  is 
gained  in  force  by  a  machine  is  lost  in  distance  or  in  time  ;  but,  by 
modifying  its  action,  they  render  it  capable  of  performing  work 
which  it  alone  could  not  do.  For  instance,  by  the  aid  of  a  lever,  a 
man  can  raise  burdens,  which,  without  such  help,  would  be  impos- 
sible. We  shall  only  describe  here  the  lever,  the  simplest  of  all 
machines,  and  shall  afterwards  see  its  action  in  the  case  of  balances. 

33.  levers. — A  lever  is  a  rigid  bar  of  wood  or  of  metal  mov- 
able about  a  fixed  point  or  edge  called  \b.e  fulcrum ;  and  subject  to 
the  action  of  two  forces  which  tend  to  move  it  in  opposite  direc- 
tions. The  force  which  acts  as  motor  is  called  the  power,  and  the 
other  the  resistance.  Levers  are  divided  into  three  classes,  accord- 
ing to  the  different  positions  of  the  power,  and  resistance  in 
reference  to  the  fulcrum. 

A  lever  of  the  first  kind'vs,  one  in  which  the  fulcrum  is  between 
the  power  and  the  resistance.  Fig.  17  represents  one  of  this  kind, 
in  which  the  hand  is  the  power,  the  weight  P  the  resistance,  while 
C  is  the  fulcrum. 


Fig.  17. 

A  lever  of  the  second  kind\a&  the  resistance  between  the  power 
and  the  fulcrum,  as  in  fig.  18. 

A  lever  of  the  third  kind  is  one  in  which  the  power  is  applied 
between  the  resistance  and  the  fulcrum  as  represented  in  fig.  19. 

In  these  different  kinds  of  levers,  the  distances  from  the  fulcrum 


26    Properties  of  Matter  and  Universal  Attraction.  [33- 

to  the  power  and  to  the  resistance  are  called  the  arms  of  the  lever. 
In  fig.  19,  for  instance,  the  arm  of  the  power  is  the  distance  from  C 
to  B,  and  that  from  C  to  A  is  the  arm  of  the  resistance. 


Fig. 


Fig.  19- 


34.  Effect  of  levers.     Condition  of  equilibrium. — It  may  be 

shown  that  the  effect  produced  by  a  force  by  means  of  a  lever  in- 
creases with  the  length  of  the  arm  upon  which  it  acts,  that  is,  if  the 


35] 


Applications  of  Levers. 


27 


arm  is  twice,  thrice,  or  four  times  as  long,  the  useful  effect  is  two, 
three,  or  four  times  as  great.  This  is  what  led  Archimedes  to  say, 
that,  give  him  a  fulcrum,  and  he  would  lift  the  world. 

Since  a  force  produces  a  greater  effect  the  longer  the  arm  of  the 
lever,  it  follows  that  in  order  to  produce  equilibrium  between  the 
power  and  the  resistance,  acting  at  the  same  time  on  a  lever,  if  the 
arms  are  equal,  the  two  forces  themselves  must  be  equal,  and  that  if 
the  arms  of  the  lever  are  unequal,  the  two  forces  must  b€  inversely 
as  the  arms  of  the  lever  \  thus,  if  the  power  is  one-third  that  of  the 
resistance,  the  arm  of  the  power  should  be  three  times  as  long  as 
that  of  the  resistance. 

In  a  lever  of  the  third  kind  the  power  must  be  always  greater 
than  the  resistance,  for  the  distance  of  the  resistance  from  the  ful- 
crum (AC,  fig.  19)  is  always  greater  than  the  distance  BC  from  the 
power  B  to  the  fulcrum.'  In  a  lever  of  the  second  kind  the  power 
is  always  smaller  than  the  resistance,  for  the  arm  BC  is  longer  than 
the  arm  AC  (fig.  18).  These  properties  are  expressed  by  saying 
that,  in  a  lever  of  the  third  kind  there  is  a  loss  of  power,  and  in  one 
of  the  second  kind  a  gain.  In  a  lever  of  the  first  kind  there  may  be 
either  gain,  or  loss,  or  they  may  just  balance  each  other,  for  the  arm 
BC  of  the  power  (fig.  17)  may  be  either  greater,  or  less  than,  or 
equal  to,  the  arm  AC. 

35.  Various  applications  of  levers. — Numerous  applications 
of  the  different  kinds  of 
levers  are  met  with  in 
articles  of  every-day  use. 
The  ordinary  balance 
(fig.  ,34)  is  a  lever  of  the 
first  kind,  as  is  also  a  Fl§- 20- 

pump  handle.  Scissors  are  another  instance  ;  each  handle  is  a 
lever,  the  fulcrum  of  which  is  the  pivot  C,  the  power  is  the  hand 
and  the  resistance  is  the  material  to  be  cut  (fig.  20). 

As  levers  of  the  second  class  may  be  enumerated  the  oars  of  a 
boat.     The  resistance  of  the  water  to  the  motion  of  the  feather  of 
the    oar   represents  the   ful- 
crum, the  hand  of  the  oars- 
man is  the  power,  and  the 
boat,  or  rather  the  water  it 
displaces,  is  the   resistance. 
The  knife  fixed  at  one  end  Fie- «• 

and  used  in  slicing  roots,  or  cutting  bread,  is  a  lever  of  the  second 


28    Properties  of  Matter  and  Universal  Attraction.  [35- 

kind.  Nutcrackers  (fig.  21)  afford  a  third  illustration,  as  also  does 
the  common  wheelbarrow. 

When  two  porters  carry  on  a  pole  a  load  placed  midway  be- 
tween them,  they  share  it  equally,  that  is,  each  bears  half,  for  the 
pole  becomes  a  lever,  of  which  each  porter  is  a  fulcrum  as  regards 
the  other ;  but  if  the  load  be  nearer  one  than  the  other,  he  to 
whom  it  is  nearer  bears  proportionally  more  of  its  weight. 

The  consideration  of  this  kind  of  lever  explains  why  a  finger, 
caught  near  the  hinge  of  a  shutting  door,  is  so  severely  crushed. 

The  third  kind  of  lever  is  less  frequently  met  with.  The  pedals 
used  in  pianos  and  in  grindstones  are  instances.  In  the  latter  case 
the  pedal  consists  of  a  wooden  board  AC  (fig.  22)  forming  a  lever. 


Fig.   22. 

The  fulcrum  is  at  C  on  a  bolt  fixed  to  the  frame  ;  the  power  is  the 
foot  of  the  man  turning,  and  the  resistance,  which  is  the  motion  to 
be  transmitted  to  the  wheel,  is  applied  at  A  by  means  of  a  rod  joined 
to  a  crank  in  the  centre  of  the  mill. 

In  the  common  fire-tongs  each  leg  is  a  lever  of  the  third  kind. 
The  hand  of  a  man  pushing  open  a  gate  while  standing  near  the 


.-37]  -Gravitation.  29 

hinges  moves  through  much  less  space  than  the  end  of  the  gate, 
and  must  exert,  therefore,  a  proportionally  greater  force. 

The  most  beautiful  and  numerous  instances  are  met  with  in  the 
muscular  system  of  men  and  animals,  almost  all  motions  of  which 
are  effected  by  this  mechanism. 


CHAPTER   IV. 

36.  Universal  attraction. — It  is  stated  that  Newton,  sitting  one 
day  in  his  garden  and  seeing  an  apple  fall  from  a  tree,  was  led  by 
this  circumstance  to  reflect  upon  the  cause  why  bodies  fell  to  the 
ground,  and  ultimately  to  the  discovery  of  the  important  laws  which 
govern  the  motion  of  the  earth  and  of  the  stars. 

They  may  be  thus  stated  : 

-  i.  All  bodies  in  nature  exert  a  mutual  attraction  upon  each  other 
at  all  distances,  in  'virtue  of  which  they  are  continually  tending 
towards  each  other. 

2.  For  the  same  distance  the  attractions  between  bodies  are  pro- 
portional to  their  masses. 

3.  The  masses  being  equal,  the  attraction  varies  with  the  distance, 
being  inversely  proportional  to  the  square  of  the  distances  asunder. 

To  illustrate  this,  we  may  take  the  case  of  two  spheres  which, 
owing  to  their  symmetry,  attract  each  other  just  as  if  their  masses 
were  concentrated  in  their  centres.  If  without  other  alteration  the 
mass  of  one  sphere  were  doubled,  trebled,  etc.,  the  attraction  be- 
tween them  would  be  doubled,  trebled,  etc.  If,  however,  the  mass 
of  one  sphere  being  doubled,  that  of  the  other  were  increased  three 
times,  the  distance  between  their  centres  remaining  the  same,  the 
attraction  would  be  increased  six  times.  Lastly,  if,  without  altering 
their  masses,  the  distance  between  their  centres  were  increased 
from  i  to  2,  3, 4,  .  .  .  units,  the  attraction  would  be  diminished  the 
4th,  9th,  1 6th  .  .  .  part  of  its  former  intensity. 

37.  Gravitation. — The  term  gravitation  is  applied  more  es- 
pecially to  the   attraction  exerted  between  the  heavenly  bodies. 
The  sun,  being  that  member  of  our  planetary  system  which  has  the 
largest   mass,  exerts   also   the  greatest  attraction,  from  which  it 
might  seem  that  the  earth  and  the  other  planets  ought  to  fall  into 
the  sun  in  virtue  of  this  attraction.      This  would  indeed  be  the  case 


3O    Properties  of  Matter  and  Universal  Attraction.  [37- 

if  they  were  only  acted  upon  by  the  force  of  gravitation  ;  but  owing 
to  their  inertia,  the  original  impulse  which  they  once  received,  con- 
stantly tends  to  carry  them  away  from  the  sun  in  a  straight  line. 
This  acquired  velocity,  combined  with  gravitation,  makes  the  planets 
describe  curves  about  the  sun  which  are  almost  circular,  and  are 
called  their  orbits. 

38.  Gravity. — This  is  the  force  in  virtue  of  which  bodies  fall 
when   they  are   no   longer   supported,  that   is,  tend  towards  the 
centre  of  the  earth.     It  is  a  particular  case  of  universal  attraction  ; 
and  is  due  to  the  reciprocal  attraction  exerted  between  the  earth 
and  bodies  placed  on  its  surface  :  it  acts  equally  upon  all  bodies, 
whether  they  are  at  rest  or  in  motion  ;  whether  they  are  solids, 
liquids,  or  gases.     Some  bodies,  such  as  clouds  and  smoke,  appear 
not  to  be   influenced  by  this  force,  for  they  rise   in   the   atmo- 
sphere instead  of  sinking  ;   yet  this,  as  will  afterwards  be  seen, 
is  no  exception  to  the  action  of  gravity. 

Gravity,  being  a  particular  case  of  universal  attraction,  acts 
upon  bodies  proportionally  to  their  mass  and  inversely  as  the  square 
of  their  distance  ;  that  is,  a  body  which  contains  twice  or  thrice  as 
much  matter  as  another,  is  attracted  by  the  earth  with  a  twofold  or 
threefold  force  ;  or,  in  other  words,  weighs  twice  or  thrice  as  much. 
In  like  manner  if  one  and  the  same  body  could  be  moved  to  twice  or 
thrice  its  present  distance  from  the  centre  of  the  earth,  it  would  have 
one-fourth  or  one-ninth  of  its  present  weight ;  we  say  the  centre  and 
not  the  surface  of  the  earth,  for  it  is  demonstrated  in  treatises  on 
mechanics  that  the  attractive  force  of  the  earth  which  causes 
bodies  to  fall  must  be  calculated  from  its  centre. 

From  the  magnitude  of  the  earth's  radius,  which  is  about  4,000 
miles,  all  bodies  on  its  surface  may  be  considered  to  be  virtually  at 
the  same  distance  from  the  centre,  and  we  may  therefore  conclude 
that  their  difference  in  weight  is  merely  due  to  their  difference  in  mass. 

39.  The  weight  of  a  body  increases  from  the  equator  to  the 
poles. — The  force  which  makes  bodies  fall  is  not  exactly  the  same 
at  all  points  of  the  earth's  surface.     Two  causes  make  it  increase 
from  the  equator  to  the  poles  :  the  daily  rotation  of  the  earth  about 
its  axis,  and  the  flattening  at  the  poles.     For  the  rotation  of  the 
earth  gives  rise  to  a  centrifugal  force  acting  from  the  centre  to  the 
surface,  that  is,  in  the  opposite  direction  to  the  force  of  gravity. 
Hence  bodies  are  continually  acted  upon  by  two  forces  in  opposite 
directions ;    the  force  of  gravity  which  draws  them  towards  the 
centre,  and  the  centrifugal  force  which  tends  to  drive  them  away 


-40] 


Weight  of  a  Body. 


from  it.  So  that  it  is  really  the  excess  of  the  first  force  over  the 
second  which  makes  bodies  fall.  But  as  the  centrifugal  force  de- 
creases from  the  equator  to  the  poles  (30),  the  excess  of  gravity  over 
this  force  becomes  greater,  and  thus  the  weights  of  bodies  increase 
as  they  come  nearer  the  poles. 

The  flattening  of  the  earth  concurs  in  producing  the  same  effect ; 
for,  in  consequence  of  it,  bodies  placed  on  the  surface  of  the  earth 
are  nearer  the  centre  at  the  poles  than  they  are  at  the  equator,  and 
are  therefore  more  attracted.  It  must  be  added,  that  the  increase 
in  weight  due  to  these  two  causes  is  very  small,  and  is  inappreciable 
by  ordinary  balances. 

40.  Vertical  and  horizontal  lines. — At  any  point  of  the  earth's 
surface,  the  direction  of  gravity,  that  is,  the  line  which  a  falling 
body  describes,  is  called  the  vertical  line.  The  vertical  lines  drawn 
at  different  points  of  the  earth's  surface  converge  very  nearly 
to  the  earth's  centre.  Hence,  owing  to  the  great  distance  from 
the  surface  of  the  earth,  to  its  centre,  for  points  on  the  surface  a  and 


Fig.  23. 

&/(fig.  23),  not  far  apart,  these  verticals  may  be  assumed  to  be 
parallel ;  but  they  are  less  parallel  the  further  apart  the  points,  as 
shown  by  the  verticals  a  and  d.  For  points  situated  on  the  same 
meridian  the  angle  contained  between  the  vertical  lines  equals  the 
difference  between  the  latitudes  of  those  points. 

At  each  point  on  the  surface  of  the  earth  a  man  standing  up- 
right is  in  the  direction  of  the  vertical.  But,  as  we  have  just  seen, 
this  direction  changes  from  one  place  to  another,  and  the  same  is 
the  case  with  the  position  of  the  inhabitants  of  the  various  countries 
on  the  earth.  As  the  earth  is  spherical,  it  follows  that  at  two  points, 
exactly  opposite,  two  men  will  be  in  inverted  positions  in  reference 


32    Properties  of  Matter  and  Universal  Attraction.  [40- 

to  each  other ;  from  which  is  derived  the  term  antipodes  (opposite 
as  regards  the  feet),  given  to  the  inhabitants  of  two  diametrically 
opposite  places. 

A  plane  or  a  line  is  said  to  be  horizontal  when  it  is  perpendicular 
to  the  direction  of  the  vertical.  The  surface  of  water  in  a  state  of 
equilibrium  is  always  horizontal.  In  speaking  of  the  level  we  shall 
learn  how  the  horizontally  of  any  surface  or  line  is  determined. 

41.  Plumb-line. — The  vertical  line  at  any  point  of  the  globe  is 
generally  determined  by  the  plumb-line  (fig.  24),  which  consists  of 
a  cylindrical  weight  attached  to  the  end  of  a  string.  In  obedience 
to  the  action  of  gravity  this  weight  draws  the  string  in  the  direction 
of  this  force,  and  when  it  is  at  rest  the  string  is  in  the  vertical  direc- 
tion. To  ascertain  by  aid  of  the  plumb-line  whether  a  given 
surface,  a  wall  for  example,  is  vertical,  a  small  metal  plate  is  used., 


the  side  of  which  is  equal  to  the  diameter  ot  the  weight.  In  the 
centre  of  this  plate  is  a  small  hole,  through  which  passes  the 
string  :  holding  in  one  hand  the  plate,  and  in  the  other  the  string, 
the  edge  of  the  plate  is  placed  against  the  wall  (fig.  24) ;  if  the 
weight  just  touches  it  the  wall  is  vertical ;  if  the  cylinder  does  not 
touch  the  wall,  it  shows  that  the  wall  is  inclined  outwards  ;  it  is 
inclined  inwards  if  the  weight  touches  the  wall  when  the  plate  is  a 
little  removed  from  it. 


-44]        Determination  of  the  Centre  of  Gravity.  3  3 

42.  Weight  of  a  body. — The  weight  of  a  body  is  the  sum  of 
the  partial  attractions  which  the  earth  exerts  upon  each  of  its  mole- 
cules.   Hence  the  weight  of  a  body  must  increase  as  its  mass  does  ; 
that  is,  if  it  contains  twice  or  thrice  as  much  matter,  its  weight 
must  be  twice  or  thrice  as  great.     The  weight  of  a  body  is  not  to 
be  confounded  with  gravity ;  this  is  the  cause  which  produces  the 
fall  of  bodies  ;  the  weight  is  only  the  effect.     We  shall  presently 
see  how  weight  is  determined  by  means  of  the  balance  ;  gravita- 
tion is  measured  by  the  aid  of  the  pendulum. 

43.  Centre  of  gravity. — We  have  seen  that  all  the  partial  attrac- 
tions which  the  earth  exerts  upon  each  of  the  molecules  of  a  body  are 
equivalent  to  a  single  force  which  is  the  weight  of  the  body.     Now 
it  may  be  shown  in  mechanics,  that  whatever  be  the  shape  of  any 
body,  there  is  always  a  certain  point  through  which  this  single  force, 
the  weight  acts,  in  whatever  position  the  body  be  placed  in  respect 
to  the  earth  ;  this  point  is  called  the  centre  of  gravity  of  the  body. 

To  find  the  centre  of  gravity  of  a  body  is  a  purely  geometrical 
problem  ;  in  many  cases,  however,  it  can  be  at  once  determined. 
For  instance,  the  centre  of  gravity  of  a  right  line  of  uniform 
density  is  the  point  which  bisects  its  length  ;  in  the  circle  and 
sphere  it  coincides  with  the  geometrical  centre  ;  in  cylindrical  bars 
it  is  the  middle  point  of  the  axis  ;  in  a  square  or  a  parallelogram 
it  is  at  the  point  of  intersection  of  the  two  diagonals.  These  rules, 
it  must  be  remembered,  presuppose  that  the  several  bodies  are 
of  uniform  density. 

44.  Experimental  determination  of  the  centre  of  gravity. — 
The  centre  of  gravity  of  a  body  may  also  be  found  by  experiment. 


Fig.  25. 

When  its  weight  is  not  too  great  it  is  suspended  by  a  string  in 
two  different  positions  ;  the  centre  of  gravity  of  the  body  is  neces- 
sarily below  the  point  of  suspension,  and  therefore  in  the  prolonga- 

I) 


34   Properties  of  Matter  and  Universal  Attraction.    [44- 

tion  of  the  vertical  cord  which  sustains  it.  If  then,  in  two  dif- 
ferent positions,  the  vertical  lines  of  suspension  be  prolonged,  they 
cut  one  another,  and  the  point  of  intersection  is  the  centre  of 
gravity  sought. 

In  the  case  of  thin  flat  substances,  like  a  piece  of  cardboard  or 
a  sheet  of  tin  plate,  the  centre  of  gravity  may  be  found  by  balancing 
the  body  in  two  different  positions  on  a  horizontal  edge  ;  for  in- 
stance sliding  them  near  the  edge  of  a  table  until  they  are  ready 
to  turn  in  either  direction  (fig.  25).  The  centre  of  gravity  is  then 
on  the  line  ab.  Seeking,  in  a  similar  manner,  a  second  position  of 
.equilibrium  in  which  the  line  of  contact  is  cd  for  instance,  the 
centre  of  gravity  must  necessarily  be  on  both  these  lines  ;  that  is, 
must  be  at  the  point  of  their  intersection^  ;  or,  more  accurately,  a 
little  below  this  point,  in  the  interior  of  the  body,  and  at  an  equal 
distance  from  its  two  faces. 

If  the  body  be  thicker,  three  positions  of  equilibrium  must  be 
found ;  the  centre  of  gravity  is  then  at  the  point  of  intersection 

of  the  three  planes  passing 
vertically  through  the  lines 
of  contact  when  the  body  is 
in  equilibrium. 

45.  Equilibrium  of  heavy 
bodies. — As  the  centre  of 
gravity  is  the  point  where  the 
whole  action  of  gravity  is 
concentrated,  it  follows  that 
whenever  this  point  rests 
upon  any  support,  the  action 
of  gravity  is  destroyed,  and 
therefore  the  body  remains 
in  equilibrium.  There  are, 
however,  several  cases,  ac- 
cording as  the  body  has  one 
or  more  points  of  support. 

Where  the  body  has  only 
one  point  of  support  equi- 
librium is  only  possible  when 

*lg-  26-  the  centre  of  gravity  either 

coincides  with  this  point,  or  is  exactly  above  or  below  it  in  the 
same  vertical  line  ;  for  then  the  action  of  gravity  is  destroyed  by 
the  resistance  of  the  fixed  point  through  which  this  force  passes. 


-46] 


Different  States  of  Equilibrium. 


35 


Fig.  27. 


The   plumb-line   (fig.    24)   is  a  case   of   this  kind,  the  centre   ot 

gravity  being  below  the  point  of  support.      Another   example  Js 

the  case  of  a   stick  balanced  on   the 

ringer,    as    seen  in  fig.    26,  in  which 

the  letter  g  indicates  the  position  of 

equilibrium  exactly  over  the  point  of 

support. 

If  the  body  has  two  points  ot 
support,  it  is  not  necessary  for  equi- 
librium that  its  centre  of  gravity  coin- 
cide with  either  of  these  points,  or  be 
exactly  above  or  below  :  it  is  sufficient 
if  it  be  exactly  below  or  above  the 
right  line  which  joins  these  two  points, 
for  the  action  of  gravity  may  then 
be  decomposed  into  two  forces  ap- 
plied at  the  points  of  support,  and 
destroyed  by  the  resistance  of  these 
points.  A  man  on  stilts  (fig.  27) 
is  an  example  of  this  case  of  equi- 
librium. 

Lastly  if  a  body  rests  on  the  ground  by  three  or  more  points  ot 
support  (fig.  28),  equilibrium  is  produced  whenever  the  centre  of 
gravity  is  above  the  base  formed 
by  these  points  of  support,  that 
is,  whenever  the  vertical  let  fall 
from  the  centre  of  gravity  to 
the  earth  is  within  the  points 
of  support  ;  for  gravity  cannot 
then  overturn  the  body  beyond 
its  points  of  support,  and  its  only 
effect  is  to  settle  it  more  firmly 
on  the  ground. 

46.  Different  states  of  equi- 
librium.—  Although  a  body 
supported  by  a  fixed  point  is 
in  equilibrium  whenever  its  ^=Ssa 

centre  of  gravity  is  in  the  vertical  Fis-  28- 

line  through  that  point,  the  fact  that  the  centre  of  gravity  tends 
incessantly  to  occupy  the  lowest  possible  position  leads  us  to  distin- 
guish between  three  states  of  equilibrium— stable,  unstable,  neutral. 

D  2 


36    Properties  of  Matter  and  Universal  Attraction.    [46- 

A  body  is  said  to  be  in  stable  equilibrium  if  it  tends  to  return 
to  its  first  position  after  the  equilibrium  has  been  slightly  disturbed. 
Every  body  is  in  this  state  when  its  position  is  such  that  the 
slightest  alteration  of  the  same  elevates  its  centre  of  gravity  ;  for 
the  centre  of  gravity  will  descend  again  when  permitted,  and  after 
a  few  oscillations  the  body  will  return  to  its  original  position. 

The  pendulum  of  a  clock  continually  oscillates  about  its  position 
of  stable  equilibrium,  and  an  egg  on  a  level  table  is  in  this  state 
when  its  long  axis  is  horizontal.  We  have  another  illustration  in 
the  toy  represented  in  fig.  29. 

These  little  figures,  which  are  hollow  and  light,  are  loaded  at  the 
base  with  a  small  mass  of  lead,  so  that  the  centre  of  gravity  is  very 


Fig.  29. 

low.  Hence  when  the  figure  is  inclined,  the  centre  of  gravity  is 
raised,  and  gravity  tending  to  make  it  descend,  the  figure  reverts 
to  its  original  position  after  a  number  of  oscillations  on  the  right 
and  left  of  its  final  position  of  eo1uilibrium. 

A  body  is  said  to  be  in  imstable  equilibrium,  when,  after  the 
*  slightest  disturbance,  it  tends  to  depart  still  more  from  its  original 
position.  A  body  is  in  this  state  when  its  centre  of  gravity  is 
vertically  above  the  point  of  support,  or  higher  than  it  would  be  in 
any  adjacent  position  of  the  body.  An  egg  standing  on  its  end, 
or  a  stick  balanced  upright  on  the  finger,  is  in  this  state  (fig.  26). 
As  soon  as  the  stick  is  out  of  the  vertical  its  centre  of  gravity  de- 
scends, and  gravity  acting  with  increasing  force,  the  stick  falls,  if 
care  be  not  taken  to  bring  the  point  of  support  below  the  centre 
,-.  of  gravity,  by  which  equilibrium  is  restored. 

Neutral  equilibrium.    A  body  is  in  a  state  of  neutral  equilibrium 


-47]  Examples  of  Stable  Equilibrium.  37 

when  it  remains  at  rest  in  any  position  which  can  be  given  to  it. 
This  can  only  be  the  case  when  an  alteration  in  the  position  of  the 
body  neither  raises  nor  lowers  its  centre  of  gravity.  A  perfect 
sphere  resting  on  a  horizontal  plane  is  in  this  state. 

Fig.  30  represents  three  cones  A,  B,  C,  placed  respectively  in 
stable,  unstable,  and  neutral  equilibrium  upon  a  horizontal  plane. 
The  letter  £-  in  each  shows  the  position  of  the  centre  of  gravity. 


Fig.  30. 

47.  Examples  of  stable  equilibrium. — From  what  has  been 
said  it  follows,  that  the  wider  the  base  on  which  a  body  rests  the 
greater  is  its  stability ;  for  then,  even  with  a  considerable  inclination, 
its  centre  of  gravity  is  above  its  base. 

The  well-known  leaning  towers  of  Pisa  and  Bologna,  are  so  much 
out  of  the  vertical  that  they  seem  ready  to  fall  at  any  moment ;  and 
yet  they  have  remained  for  centuries  in  their  present  position,  be- 
cause their  centres  of  gravity  are  above  the  base.  Fig.  31  repre- 
sents the  tower  of  Bologna,  built  in  the  year  1112,  and  known  as  the 
Garisenda.  Its  height  is  165  feet,  and  it  is  7  or  8  feet  out  of  the 
vertical.  The  leaning  is  due  to  the  foundations  having  given  way. 
The  tower  on  the  side  is  that  of  Asarelli,  the  highest  in  Italy. 

In  the  cases  we  have  hitherto  considered,  the  position  of  the 
centre  of  gravity  is  fixed  :  this  is  not  the  case  with  men  and 
animals,  whose  centre  of  gravity  is  continually  varying  with  their 
attitudes,  and  with  the  loads  they  support. 

When  a  man,  not  carrying  anything,  stands  upright,  his  centre 
of  gravity  is  about  the  middle  of  the  lower  part  of  the  pelvis,  that 
is,  between  the  two  thigh  bones.  This,  however,  is  not  the  case 
with  a  man  carrying  a  load,  for  his  own  weight  being  added  to  that 
of  the  load,  the  common  centre  of  gravity  is  neither  that  of  the  man 
nor  of  his  burden. 

In  this  case,  in  order  to  retain  his  stability,  the  man  must  so 
modify  his  attitude  as  to  keep  his  centre  of  gravity  above  the  base 


38    Properties  of  Matter  and  Universal  Attraction.    [47- 

formed  by  his  two  feet.  Thus  a  porter  with  a  load  on  his  back 
is  obliged  to  lean  forward  (fig.  32),  while  a  man  carrying  a 
load  in  one  hand  is  obliged  to  lean  his  body  on  the  opposite  side 

(fig.  33)- 

Again,  it  is  impossible  to  stand  on  one  leg  if  we  keep  one  side 


Fig.  31. 

of  the  foot  and  head  close  to  a  vertical  wall,  because  the  latter  pre- 
vents us  from  throwing  the  body's  centre  of  gravity  vertically  above 
the  supporting  base. 

In  the  art  of  rope-dancing  the  difficulty  consists  in  maintaining 
the  centre  of  gravity  exactly  above  the  rope.  In  order  more  easily 
to  accomplish  this  the  performer  holds  in  his  hands  a  long  pole, 
which,  as  soon  as  he  feels  himself  leaning  on  one  side,  he  inclines 
towards  the  opposite  one  ;  and  thus  contrives  to  keep  the  centre  of 


-48] 


The  Balance. 


39 


gravity  common  to  himself  and  to  the  pole,  in  a  vertical  line  above 
the  rope,  and  so  preserves  his  equilibrium. 


Fig.   32.  Fig.  33. 

48.  The  balance. — The  balance  is  an  instrument  for  deter- 
mining the  relative  weights  or  masses  of  bodies.  There  are  many 
varieties. 

The  ordinary  balance  (fig.  34)  consists  of  a  lever  of  the  first  kind, 
called  the  beam,  with  its  fulcrum  in  the  middle  ;  at  the  extremities  of 
the  beam  are  suspended  two  scale-pans,  D  and  C  ;  one  intended  to 
receive  the  object  to  be  weighed,  and  the  other  the  counterpoise. 
The  fulcrum  consists  of  a  steel  prism,  n,  commonly  called  a  knife- 
edge,  which  passes  through  the  beam,  and  rests  with  its  sharp  edge, 
or  axis  of  suspension,  upon  two  supports ;  these  are  formed  of  agate 
or  polished  steel,  in  order  to  diminish  the  friction.  A  needle  or 
pointer  is  fixed  to  the  beam,  and  oscillates  with  it  in  front  of  a 
graduated  arc,  a ;  when  the  beam  is  perfectly  horizontal,  the  needle 
points  to  the  zero  of  the  graduated  arc. 

Since  (34)  two  equal  forces  in  a  lever  of  the  first  kind  cannot 
be  in  equilibrium  unless  their  leverages  are  equal,  the  length  of 
the  arms  ;/A  and  ?zB  ought  to  remain  equal  during  the  process  of 
weighing.  To  secure  this  the  scales  are  suspended  from  hooks, 
whose  curved  parts  have  sharp  edges,  and  rest  on  similar  edges  at 
the  ends  of  the  beam.  In  this  manner  the  scales  are  supported  on 
mere  points,  which  remain  unmoved  during  the  oscillations  of 
the  beam.  This  mode  of  suspension  is  represented  in  the  follow- 
ing figure. 


4O   Properties  of  Matter  and  Universal  Attraction.    [48- 

The  weight  of  any  body  is  determined  by  placing  it  in  one  of 
the  pans  of  the  balance  D,  for  instance,  and  adding  weights  to  the 
other  until  equilibrium  is  established,  which  is  the  case  when  the 
beam  is  quite  horizontal. 


Fig.  34- 

49.  Conditions  to  be  satisfied  by  a  good  balance. — A  good 
balance  should  be  accurate,  that  is,  it  should  give  exactly  the  weight 
of  a  body  :  it  should  also  be  delicate,  that  is,  the  beam  should  be 
inclined  by  a  very  small  difference  between  the  weights  in  the  two 
scales. 

Conditions  of  accuracy,     i.    The  two  arms  of  the  beam  ought  to 


50]  Method  of  Double  Weighing.'  41 

be  precisely  equal,  otherwise,  according  to  the  principle  of  the  lever 
(34),  unequal  weights  will  be  required  to  produce  equilibrium.  To 
test  whether  the  arms  of  the  beam  are  equal,  weights  are  placed  in 
the  two  scales  until  the  beam  becomes  horizontal ;  the  contents  of 
the  scales  being  then  interchanged,  the  beam  will  remain  horizontal 
if  its  arms  are  equal,  but  if  not  it  will  descend  on  the  side  of  the 
longer  arm. 

ii.  The  balance  ought  to  be  in  equilibrium  when  the  scales  are 
empty  \  for  otherwise,  unequal  weights  must  be  placed  in  the  scales 
in  order  to  produce  equilibrium.  It  must  be  borne  in  mind,  how- 
ever, that  the  arms  are  not  necessarily  equal,  even  if  the  beam  re- 
mains horizontal  when  the  scales  are  empty  ;  for  this  result  might 
also  be  produced  by  giving  to  the  longer  arm  the  lighter  scale. 

iii.  The  beam  being  horizontal,  its  centre  of  gravity  ought  to  be 
in  the  same  vertical  line  with  the  edge  of  the  fulcrum,  and  a  little 
below  the  latter.  For  if  the  centre  of  gravity  coincided  with  this 
line,  the  action  of  gravity  on  the  beam  would  be  null,  and  it  would 
not  oscillate.  If  the  centre  of  gravity  were  above  the  edge  of  the 
fulcrum,  the  beam  would  be  in  unstable  equilibrium  ;  while,  if  it  is 
below  the  fulcrum,  the  weight  of  the  beam  is  continually  tending  to 
bring  it  back  to  the  horizontal  position  as  soon  as  it  diverges  from 
it,  and  the  balance  oscillates  with  regularity. 

Conditions  of  delicacy.  I.  The  centre  of  gravity  of  the  beam 
should  be  very  near  the  knife  edge ;  for  then,  when  the  beam  is 
inclined,  its  weight  only  acting  upon  a  short  arm  of  the  lever,  offers 
but  little  resistance  to  the  excess  of  weight  in  one  of  the  pans. 

2.  The  beam  should  be  light ;  for  then  the  friction  of  the  knife 
edge  upon  the  supports  is  smaller  the  less  the  pressure.     In  order 
more  effectually  to  diminish  friction,  the  edges  from  which  the 
beam  and  scales  are  suspended  are  made  as  sharp  as  possible,  and 
the  supports  on  which  they  rest  are  very  hard. 

3.  Lastly,  the  longer  the  beam  the  more  delicate  is  the  balance  ; 
because  the  difference  in  the  weights  in  the  pans  then  acts  upon 
a  longer  arm  of  the  lever. 

50.  Method  of  double  weighing:. — Notwithstanding  the  inac- 
curacy of  a  balance,  the  true  weight  of  a  body  may  always  be 
determined  by  jt.  To  do  so,  the  body  to  be  weighed  is  placed  in 
one  scale,  and  shot  or  sand  poured  into  the  other  until  equilibrium 
is  produced  ;  the  body  is  then  replaced  by  known  weights  until 
equilibrium  is  re-established.  The  sum  of  these  weights  will 
necessarily  be  equal  to  the  weight  of  the  body,  for,  acting  under 


42    Properties  of  Matter  and  Universal  Attraction.   [50- 

precisely  the  same  circumstances,  both  have  produced  precisely  the 
same  effect. 

51.  Weighing  machines. — One  of  the  forms  of  these  instru- 
ments, which  are  of  frequent  use  in  railway  stations,  coal  yards, 
etc.,  for  weighing  heavy  loads,  is  represented  in  fig.  35.  It  con- 
sists of  a  platform,  A,  on  which  the  body  to  be  weighed  is  placed, 
and  to  which  an  upright  B  is  fixed  ;  the  whole  rests  on  a  frame, 
HE,  by  the  following  mode  of  suspension. 


FiS-   35- 

To  the  upright,  E,  are  adapted  two  pieces  of  iron,  which  support 
a  beam,  LR,  by  the  aid  of  a  knife  edge,  which  traverses  it  at  O. 
The  two  arms  of  the  beam  are  unequal  in  length  ;  one  of  them 
supports  a  scale,  D,  in  which  are  placed  the  weights  ;  the  other  arm 
of  the  beam  has  two  rods,  by  which  is  suspended  the  movable 
part,  AB.  In  order  to  relieve  the  knife  edge  which  supports  the 
platform,  and  to  avoid  a  shock  when  it  is  unloaded,  after  a 
weighing  has  been  made,  the  arm,  OR,  is  lifted  by  raising  a  sup- 
port, r,  which  is  below  the  beam,  by  means  of  the  handle,  M.  The 
horizontality  of  the  beam  is  ascertained  by  means  of  two  indicators, 
m  and  ;z,  the  first  fixed  to  the  frame  and  the  second  to  the  beam. 

To  understand  the  working  of  the  mechanism  reference  must 


-51] 


Weighing  Machines. 


43 


be  made  to  fig.  36,  in  which  the  principal  pieces  only  are  repre- 
sented. A  lever,  th,  which  bifurcates  underneath  the  platform, 
rests  at  one  end  on  a  double  knife  edge,  z',  and  at  the  other,  on  the 
lower  end  of  the  rod,  L//,  which  is  fixed  to  the  beam.  A  second 
lever,  eg,  rests  at  s  on  the  lever  z'/z,  attached  at  g  to  the  rod  ag, 
which  is  also  supported  by  the  beam.  Lastly,  the  distance  is  being 
the  fifth  of///,  aO  is  also  a  fifth  of  OL. 

From  this  division  of  the  two  levers,  ih  and  OL  into  propor- 
tional parts  two  important  consequences  follow.  First,  that  when- 
the  beam  oscillates,  the  points  a  and  g  being  lowered  by  a  certain 
amount,  the  points  L  and  h  are  lowered  five  times  as  much.  But 


1,1. 


Fig.  36. 

for  a  similar  reason,  since  the  lever  ih  oscillates  upon  the  knife 
edge  /,  the  knife  edge  s  is  lowered  one  fifth  as  much  as  the  point, 
//,  and  therefore  just  as  much  as  g.  The  lever  ^therefore  descends 
parallel  to  itself,  and  therefore  also  the  platform  A. 

Secondly  :  it  follows  moreover,  from  the  proportional  division 
of  the  levers  OL  and  ///,  that  the  pressure  at  the  points  of  sus- 
pension, exercised  by  the  load  g  on  the  platform,  is  independent 
of  the  place  which  it  occupies  on  the  latter,  so  that  it  just  acts  as  if 
it  were  applied  along  the  rod  ag.  This  may  be  deduced  from  the 
properties  of  the  lever  by  a  simple  calculation,  which  cannot  how- 
ever be  given  here. 

Lastly,  since  the  weight  is  applied  at  a,  the  longer  the  arm  of 
the  lever  OC  as  compared  with  Qa,  the  smaller  need  be  the  weight 
in  the  scale  D,  in  order  to  produce  equilibrium.  In  most  weighing 
machines  Oa  is  the  tenth  of  OC.  Hence  the  weights  in  the  scale 
D  represent  one-tenth  the  weight  of  the  body  on  the  platform. 


44    Properties  of  Matter  and  Universal  A  ttraction,    [52- 


CHAPTRR  V 

LAWS   OF   FALLING   BODIES.      INCLINED    PLANE.      THE 
PENDULUM. 

52.  Laws  of  falling  bodies. — When  bodies  fall  in  a  vacuum- 
that  is,  when  they  experience  no  resistance — their  fall  is  subject  to 
the  following  laws  : — 

I.  In  a  vaciium  all  bodies  fall  with  equal  rapidity. 

II.  The  space  which  a  falling  body  traverses  is  proportional  to 

the  square  of  the  time  during  which  it 
has  fallen  ;  that  is,  that  if  the  space 
traversed  in  a  second  is  16  feet,  in  two 
seconds  it  will  be  64  feet  ;  that  is,  4 
times  as  much,  and  in  3  seconds  9 
times  as  much,  or  144  feet,  and  so  on. 

III.  The  velocity  acquired  by  a  fall- 
ing body  is  proportional  to  the  duration 
of  its  fall ;  that  is,  that  if  the  velocity 
at  the  end  of  a  second  is  16  feet,  at 
the  end  of  two  seconds  it  is  twice  16, 
or  32  feet,  at  the  end  of  3  seconds  48 
feet,  and  so  forth. 

To  demonstrate  the  first  law  by 
experiment  a  glass  tube  about  two 
yards  long  (fig.  37)  may  be  taken, 
having  one  of  its  extremities  com- 
pletely closed,  and  a  brass  cock  fixed 
to  the  other.  After  having  introduced 
bodies  of  different  weights  and  densi- 
ties (pieces  of  lead,  paper,  feather,  &c.) 
into  the  tube,  the  air  is  withdrawn 
from  it  by  an  air-pump,  and  the  cock 
closed.  If  the  tube  be  now  suddenly 
reversed,  all  the  bodies  will  fall  equally 


Fig.  37- 


quickly.  On  introducing  a  little  air  and  again  inverting  the  tube 
the  lighter  bodies  become  slightly  retarded,  and  this  retardation 
increases  with  the  quantity  of  air  introduced. 


-53]  Inclined  Plane.  45 

It  is,  therefore,  concluded  that  terrestrial  attraction  which  is  the 
cause  to  which  the  fall  of  bodies  is  due,  is  equally  exerted  on  all 
substances,  and  that  the  difference  in  the  velocity  with  which  bodies 
fall  is  occasioned  by  the  resistance  of  the  air  which  is  more  per- 
ceptible the  smaller  the  mass  of  bodies  and  the  greater  the  surface 
they  present. 

The  resistance  opposed  by  the  air  to  falling  bodies  is  especially 
remarkable  in  the  case  of  liquids.  The  Staubbach  in  Switzerland 
is  a  good  illustration  ;  an  immense  mass  of  water  -is  seen  falling 
over  a  high  precipice,  but  before  reaching  the  bottom  it  is  shattered 
by  the  air  into  the  finest  mist.  In  a  vacuum,  however,  liquids  fall 
like  solids,  without  separation  of  their  molecules.  The  water 
ha/nmer  illustrates  this  ;  the  instrument  consists  of  a  thick  glass 
tube  about  a  foot  long,  half  filled  with  water,  the  air  having  been 
expelled  by  ebullition  previous  to  closing  one  extremity  with  the 
blow-pipe.  When  such  a  tube  is  suddenly  inverted  the  water  falls 
in  one  undivided  mass  against  the  other  extremity  of  the  tube,  and 
produces  a  sharp  dry  sound,  resembling  that  which  accompanies 
the  shock  of  two  solid  bodies. 

The  two  other  laws  are  verified  by  the  aid  of  the  inclined  plane, 
and  of  Atwood's  machine  (iig.  40). 

53.  Inclined  plane. — Any  plane  surface  more  or  less  oblique  in 


reference  to  the  horizon  is  an  inclined  plane.  Such  is  the  surface 
(fig.  38),  and  such  also  that  of  an  ordinary  desk  and  of  most  roads. 
When  a  body  rests  on  a  horizontal  plane,  the  action  of  gravity 
is  entirely  counteracted  by  the  resistance  of  this  plane.  This, 
however,  is  not  the  case  when  it  is  placed  upon  an  inclined  plane  : 


46    Properties  of  Matter  and  Universal  Attraction.    [53- 

the  action  of  gravity  is  then  decomposed  into  two  forces  (26),  one 
perpendicular  to  the  inclined  plane,  that  is,  acting  along  its  sur- 
face, and  the  other  parallel  to  the  plane.  The  only  effect  of  the 
first  force  is  to  press  the  first  on  the  plane  without  imparting  to  it 
any  motion  ;  while  the  second  makes  the  body  descend  along  the 
plane.  This  latter,  however,  is  only  one  component  of  gravity  :  it 
is  only  a  fraction,  a  third,  or  a  quarter,  according  to  the  degree  of 
inclination  of  the  plane.  Hence  a  body  will  roll  down  an  inclined 
plane,  but  more  slowly  than  if  it  fell  vertically ;  and  the  velocity  is 
indeed  less  the  smaller  the  angle  which  the  plane  makes  with  the 
horizon. 

A  horse  drawing  a  carriage  on  a  road  where  there  is  a  rise  of 
one  in  twenty  is  really  lifting  one-twentieth  of  the  load,  besides 
overcoming  the  friction  of  the  carriage.  Hence  the  importance  of 
making  roads  as  level  as  possible  ;  it  is  for  this  reason  that  a  road 
up  a  very  steep  hill  is  made  to  wind  or  zig-zag  all  the  way,  and 
an  intelligent  driver  in  ascending  a  steep  hill  on  which  is  a  broad 
road  usually  winds  from  side  to  side. 

The  principle  of  the  inclined  plane  is  made  use  of  in  rolling 
heavy  casks  into  or  out  of  a  waggon  by  means  of  two  combined 
beams. 

54.  Demonstration  of  the  second  law  of  falling-  bodies  by 
the  inclined  plane. — The  above  property  which  the  inclined  plane 
possesses,  of  slackening  the  fall  of  bodies,  has  been  used  to  demon- 
strate the  second  law  of  their  fall  (52),  that  the  space  traversed  by  a 

falling  body  is  proportional  to  the  square  of  the  time  during  which 
it  has  been  falling. 

To  make  this  experiment,  an  inclined  plane  is  taken,  along  which 
is  traced  a  scale  graduated  in  inches  ;  then  taking  a  well-polished 
ivory  ball,  a  position  is  found  by  trial,  at  which  it  just  takes  a  second 
to  reach  the  bottom  of  the  inclined  plane  A.  Let  us  suppose  that 
this  is  at  the  eleventh  division.  The  experiment  is  then  repeated 
by  making  the  ball  traverse  four  times  the  distance,  that  is,  placing 
it  at  the  forty-fourth  division,  and  it  will  then  be  found  to  take  two 
seconds  in  so  doing.  In  like  manner  it  will  be  found  that  in 
passing  through  nine  times  the  distance,  or  through  ninety-nine 
divisions,  three  seconds  are  required.  Hence  it  is  concluded,  that 
the  spaces  traversed  increase  as  the  squares  of  the  times. 

55.  Atwood's  machine. — Mr.  Atwood  invented  a  machine  by 
which  the  velocity  of  falling  bodies  is   slackened,  and  the   laws 
of  motion  may  be  demonstrated.     It  consists  of  a  wooden  pillar 


-55]  Atwood's  Mac/tine.  47 

about  2\  yards  high  (fig.  40).  On  the  front  of  the  pillar  is  a  clockwork 
motion,  H,  regulated  in  the  usual  way  by  a  seconds'  pendulum,  P. 
On  the  right  of  the  column  is  a  graduated  scale  which  measures 
the  spaces  traversed  by  the  falling  bodies.  Along  this  scale  two 
sliders  move,  which  can  be  fixed  by  a  screw  in  any  position  ;  one  of 
these  has  a  disc,  A,  and  the  other  a  ring,  B  (fig.  44).  At  the  top 
of  the  column  is  a  brass  pulley  whose  axis  instead  of  resting  on 
pivots,  turns  on  four  other  wheels, 
r,  r,  rf,  r',  called  friction  wheels, 
since  they  serve  to  diminish  fric- 
tion (fig.  39).  Two  exactly  equal 
weights,  K  and  K',  are  attached  to 
the  end  of  a  fine  silk  thread,  which 
passes  round  the  pulley. 

At  the  top  of  the  column  is  a 
plate,  «,  on  which  is  placed  the 
falling  body  (fig.  41).  This  plate 
is  fixed  to  a  horizontal  axis  which 
carries  a  small  catch  /,  supported,  Flg<  39> 

when  the  plate  is  horizontal,  by  a  lever,  ab,  movable  in  the  middle. 
A  spring  placed  behind  the  dial  tends  to  keep  this  lever  in  the 
position  represented  in  fig.  41,  while  an  excentric,  e,  moved  by  the 
clockwork,  tends  to  incline  towards  the  right  the  upper  arm  of  the 
lever  ad.  The  parts  are  so  arranged,  that  when  the  needle  is  at 
zero  of  the  graduation,  the  lever  ab  is  moved  by  the  excentric  ; 
the  plate  n  then  lets  fall  the  body  which  it  sustained  (fig.  42). 

These  details  being  premised,  we  may  add  that  the  slackening 
which  it  produces  in  the  fall  of  a  body  depends  on  the  mechanical 
principle  that  when  a  moving  body  meets  another  at  rest  it  imparts 
to  this  latter  a  part  of  its  velocity,  which  is  greater  the  greater  the 
mass  of  the  second  body  compared  with  the  first.  For  instance  if 
a  body  with  the  mass  i,  strikes  against  another  at  rest  with  the 
mass  19,  the  total  mass  being  now  20,  the  common  velocity  after 
the  impact  is  only  a  twentieth  of  the  original  velocity  of  the  first. 

First  experiment.  To  demonstrate  the  second  law,  that  the  spaces 
traversed  are  proportional  to  the  squares  of  the  times ,  a  weight  K 
is  placed  upon  the  ledge  n  (fig.  41),  and  it  is  loaded  with  an  over- 
weight, which  consists  of  a  brass  disc,  m  (fig.  47,)  open  at  the  side, 
so  as  to  let  pass  a  rod  fixed  to  the  weight  K.  Then  below  the  ledge 
n  the  slider  A  is  placed  at  such  a  distance  that  Km  requires  a 
second  to  traverse  the  space  n  A,  which  is  easily  obtained  after  a 


48    Properties  of  Matter  and  Universal  Attraction.    [55 


Fig.  41.  Fig.  42-  Fig.  43 


Fig.  46. 


Fig.  47- 


Fig.  40. 


-55]  Atwood's  Machine.  49 

few  trials.  If  the  mass  ;;/  fell  alone  it  would  traverse  in  a  second 
about  32  feet ;  but  from  the  principle  stated  above,  it  can  only  fall 
by  imparting  to  the  masses  K  and  K'  what  it  carries  with  it,  and 
hence  its  fall  is  the  more  diminished  the  smaller  the  mass  ;;z,  as 
compared  with  the  sum  of  the  masses  K  and  K'. 

The  experiment  being  prepared  as  indicated  in  fig.  41  the  pendu- 
lum is  made  to  oscillate ;  the  clockwork  then  begins  to  move,  and 
as  soon  as  the  needle  arrives  at  zero  the  plate  n  drops  (fig.  42),  the 
weights  K  and  m  fall  too,  and  the  space  «A  is  traversed  in  a  second 
by  a  uniformly  accelerated  motion.  The  experiment  is  recom- 
menced, the  slider  A  being  placed  at  four  times  its  original  distance, 
that  is,  that  if  the  distance  A«  were  8  inches  (fig.  41)  it  is  now  32 
inches  (fig.  43).  But  here  when  the  plate  n  drops  it  is  found  that 
the  weight  Km  requires  exactly  two  seconds  to  traverse  the  space 
A;;.  Increasing  the  space  traversed  to  72  inches  the  time  required 
for  the  purpose  is  found  to  be  three  seconds.  That  is,  that  when 
the  times  are  twice  or  thrice  as  great,  the  spaces  traversed  are  four 
or  nine  times  as  great. 

Second  experiment. — To  prove  the  law  that  the  velocities  are  pro- 
portional to  the  times,  the  experiment  is  arranged  as  shown  in  figs. 
44,  45, and  46,  that  is  the  weights  K  and  m  being  arranged  as  in 
the  first  experiment  on  the  ledge  n,  at  a  distance  of  8  inches  below 
this  the  sliding  ring  B  is  placed,  and  at  16  inches  below  the  disc  A. 
When  the  ledge  n  has  dropped,  the  weights  K  and  m  still  require 
a  second  to  fall  from  n  to  B.  But  then  the  over-weight  m  being 
arrested  by  the  ring  B  (fig.  45),  the  weight  K  only  falls  in  virtue  of 
its'acquired  velocity.  The  motion  which  was  uniformly  accelerated 
from  o  to  B  (IQ)  is  kept  uniform  from  B  to  A  ;  for  the  weight  m 
was  the  cause  of  the  acceleration,  and  this  having  ceased  to  act,  the 
acceleration  ceases.  It  is  then  found  that  the  space  0B,  equal  to  8 
having  been  traversed  in  one  second,  the  space  BA,  equal  to  16,  is 
also  traversed  in  a  second.  That  is,  16  represents  the  velocity  of 
the  uniform  motion,  which,  starting  from  the  point  B,  has  succeeded 
to  the  uniformly  accelerated  motion. 

The  experiment  is  finally  recommenced  by  placing  the  sliding- 
ring  B  at  the  distance  32  (fig.  46),  and  sliding-disc  below  B,  also  at 
the  distance  32.  The  space  0B  being  then  four  times  as  great  as 
in  fig.  44,  the  weights  K  and  m  require,  in  accordance  with  the 
second  law,  twice  the  time.  But  the  mass  m  being  again  arrested 
by  the  slider  B,  it  is  found  that  the  weight  K  falls  alone  and  uni- 
formly from  B  to  A  in  one  second.  The  number  32  from  B  to  A 

E 


50    Properties  of  Matter  and  Universal  Attraction.  [55- 


represents  then  the  velocity  acquired,  starting  from  the  point  B 
after  two  seconds  of  fall.  In  the  first  part  of  the  experiment  it  was 
ascertained  that  the  velocity  acquired  after  one  second  was  16  ; 
hence,  in  double  the  time,  the  velocity  acquired  is  double.  It  may 
be  shown,  in  like  manner,  that  after  three  times  the  time,  the 
velocity  is  trebled,  and  so  on  ;  thus  proving  the  third  law. 

56.  Pendulum. — This  is  the  name  given  to  any  heavy  mass  sus- 
pended by  a  thread  to  a  fixed  point,  or  to  any  metallic  rod  movable 
about  a  horizontal  axis.  The  ball,  ;«,  suspended  by  the  thread  cm, 
which  is  fixed  at  the  top  at  c  (fig.  48),  is  a  pendulum. 

So  long  as  the  thread  is  vertical,  which  is  the  case  when  the 
centre  of  gravity  of  the  ball  is  exactly  below  the  point  of  suspension, 
f,  the  pendulum  remains  at  rest,  for  the  action  of  gravity  is  de- 


Fig.  48.  Fig.  49.  Fig.  50. 

stroyed  by  the  resistance  at  this  point.  This  is  no  longer  the  case 
when  the  pendulum  is  removed  from  its  vertical  position  ;  when  it 
is  placed,  for  instance,  in  the  direction  en  (fig.  49).  The  ball  being 
raised,  gravity  tends  to  make  it  fall ;  it  returns  from  n  to  m,  and 
reaches  the  latter  point  with  exactly  the  velocity  it  would  have  ac- 
quired by  falling  vertically  through  the  height,  om.  The  ball, 
accordingly,  does  not  stop  at  mt  but,  in  virtue  of  its  inertia,  and  of 
its  acquired  velocity,  it  continues  to  move  in  the  direction  mp  ;  as 


-57] 


Pendulum. 


the  ball  rises,  however,  gravity,  which  had  acted  from  n  to  m  as  an 
accelerating  force,  n6w  exerts  a  retarding  action,  for  it  acts  in  a  di- 
rection contrary  to  that  of  the  motion  ;  the  motion,  accordingly, 
becomes  slower,  and  the  ball  stops  at  a  distance,  mp,  which  would 
be  exactly  equal  to  mn,  were  it  not  for  the  resistance  of  the  air, 
and  also  the  rigidity  of  the  thread,  cm,  which,  as  it  is,  offers  a 
certain  resistance  to  being  bent  about  the  point  c,  in  passing  from 
the  position  en  to  cp,  and  vice  versa. 

This  being  premised,  the  moment  the  ball  stops  at  /,  gravity 
acting  so  as  to  make  it  fall  again,  brings  it  from  p  to  m,  when, 
owing  to  its  inherent  velocity,  it  rises  in  effect  as  far  as  n,  and  so 
on  ;  a  backward  and  forward  motion  is  thus  produced  from  n  to- 
wards p,  and  from  p  towards  n,  which  may  last  several  hours. 

This  motion  is  described  as  an  oscillating  motion.  The  path  ot 
the  ball  from  n  to  p,  or  from  p  to  n, 
is  known  as  a  semi-oscillation,  a 
complete  oscillation  being  the  mo- 
tion from  n  to  p,  and  from  p  to  ;/. 
In  France  the  former  is  known  as 
a  single  oscillation,  and  the  back- 
ward and  forward  motion  as  a 
double  oscillation. 

The  extent  or  amplitude  of  the 
oscillation  is  the  distance  between 
the  extreme  positions,  en  and  cp, 
and  is  measured  by  the  arc,  pn. 

57.  Simple  and  compound 
pendulum. — A  distinction  is  made 
in  physics  between  the  simple 
and  the  compound  pendulum.  A 
simple  pendulum  would  be  that 
formed  by  a  single  material  point, 
suspended  by  a  thread  without 
weight.  Such  a  pendulum  has  only 
a  theoretical  existence ;  and  it  has 
only  been  assumed  in  order  to  arrive 
at  the  laws  of  oscillations  of  the 
pendulum  which  we  shall  presently 
describe. 

A  compound  or  physical  pendulum  may  be  denned  to  be  any 
body  which  can  oscillate  about  a  pointer  an  axis.  The  pendulum 

£  2 


Fig.  51. 


5  2    Properties  of  Matter  and  Universal  A  t traction.    [57— 

described  above  (fig.  48)  is  of  this  kind.  The  form  may  be  greatly 
varied,  but  the  most  ordinary  one  is  a  glass  or  steel  rod  (fig.  52) 
fixed  at  the  top  to  a  thin  flexible  steel  plate,  or  to  a  knife  edge  like 
that  of  the  balance  (fig.  34).  At  the  bottom  of  the  rod  is  a  heavy 
lens-shaped  mass  of  metal,  usually  of  brass,  and  known  as  the  bob. 
The  lenticular  is  preferred  to  the  spherical  form,  for  it  presents 
less  resistan.ce  to  the  air  during  each  oscillation. 

58.  Xiaws  of  the  pendulum.     Galileo. — Whatever  be  the  form 
of  the  pendulum,  ics   oscillations  always  fall  under  the  following 
laws.     The  first  of  these,  that  one  and  the  same  pendulum  makes 
its   oscillations   in   equal   times,   was  discovered  by   Galileo,   the 
celebrated  physicist  and  astronomer,  at  the  end  of  the  sixteenth 
century.     It  is  related  that  he  was  led  to  this  discovery,  while  still 
young,  by  observing  the  regular  motion  of  a  lamp  suspended  to 
the  vault  of  the  cathedral  at  Pisa.     This  property  of  the  pendulum 
has  received  the  name  of  isochronism,  from  two  Greek  words  which 
mean  equal  times,  and  such  oscillations  are  said  to  be  isochronous. 

First  law;  or,  law  of  isochronism. — The  oscillations  of  one  and 
tJie  same  pendulum  are  isochronous,  that  is,  are  effected  in  equal 
times.  This  law  is  only  perfectly  exact  when  the  oscillations  are  of 
small  amplitude,  four  or  five  degrees  at  most  j  for  a  greater  ampli- 
tude the  oscillation  is  longer. 

Second  law  ;  or,  law  of  lengths. — With  pendulums  of  different 
lengths  the  durations  of  the  oscillations  are  proportional  to  the 
square  roots  of  the  lengths  of  the  penduhims,  that  is  to  say,  that  if 
the  lengths  of  the  pendulums  are  as  1,4,  9,  16,  the  times  of  oscilla- 
tions will  be  as  i,  2,  3,  4;  these  being  the  square  roots  of  the  former 
set  of  numbers. 

Third  law. — If  the  length  of  the  pendulum  remains  the  same, 
but  the  substances  are  different,  the  duration  of  the  oscillations  is 
independent  of  the  substance  of  which  the  pendulums  are  formed ; 
that  is,  that  whether  of  wood,  or  of  ivory,  or  of  metal,  they  all 
oscillate  in  the  same  length  of  time. 

Fourth  law. —  The  duration  of  the  oscillations  of  a  given  pen- 
dulum is  inversely  as  the  square  root  of  the  force  of  gravity  in  the 
-blace  in  which  the  observation  is  made. 

59.  Verification  of  the  laws  of  the  pendulum.     In  order  to 
verify  the  laws  of  the  simple  pendulum  we  are  compelled  to  em- 
ploy a  compound  one,  the  construction  of  which  differs  as  little 
as  possible  from  that  of  the  simple  one  (57).     For  this  purpose  a 
small  sphere  of  a  very  dense  substance,  such  as  lead  or  platinum, 


-60] 


Measurement  of  tJie  Force  of  Gravity. 


53 


is  suspended  from  a  fixed  point  by  means  of  a  very  fine  thread.  A 
pendulum  thus  formed  oscillates  almost  like  a  simple  pendulum, 
the  length  of  which  is  equal  to  the  distance  of  the  centre  of  the 
sphere  from  the  point  of  suspension. 

In  order  to  verify  the  isochronism  of  small  oscillations,  it  is 
merely  necessary  to  count  the  number  of  oscillations  made  in  equal 
times,  as  the  amplitudes  of  these  oscillations  diminish  from  pn  to 
rq  (fig.  50)  say  from  three  degrees  to  a  fraction  of  a  degree  ;  this 
number  is  found  to  be  constant. 

That  the  times  of  vibration  are  proportional  to  the  square  roots 
of  the  lengths  is  verified  by  causing  pendulums,  whose  lengths  are 
as  the  numbers  i,  4,  9,  ....  to  oscillate  simultaneously.  (A  B,  fig. 
51).  The  corresponding  numbers  of  oscilla- 
tions in  a  given  time  are  then  found  to  be 
proportional  to  the  fractions  I,  £,  ^,  etc. 
.  .  .  .  ,  which  shows  that  the  times  of  os- 
cillation increase  as  the  numbers  I,  2,  3, 
....  etc. 

By  taking  several  pendulums  of  exactly 
equal  lengths  B,C,D  (fig.  51)  but  with  spheres 
of  different  substances,  lead,  copper,  ivory  ; 
it  is  found  that,  neglecting  the  resistance  of 
the  air,  these  pendulums  oscillate  in  equal 
times,  thereby  showing  that  the  accelerating 
effect  of  gravity  on  all  bodies  is  the  same 
at  the  same  place. 

60.  Measurement  of  tbe  force  of 
gravity. — The  relation  which  the  fourth  law 
of  the  pendulum  establishes  between  the 
number  of  oscillations  in  a  given  time,  and 
the  force  of  gravity,  is  used  to  determine  the 
magnitude  of  this  force  at  different  places  on 
the  globe.  By  counting  the  number  of  oscil- 
lations which  one  and  the  same  pendulum 
makes  in  a  given  time,  a  minute,  for  example, 
in  proceeding  from  the  equator  towards  the 
poles,  it  has  been  found  that  this  number 
continually  increases,  proving,  therefore,  that 
the  force  of  gravity  increases  from  the  equator  towards  the  poles. 

By  means  of  the  pendulum  the  velocity  has  been  calculated 
which  a  body  acquires  in  falling,  in  a  second  of  time,  in  vacuo,  that 


Fig.  52. 


54    Properties  of  Matter  and  Universal  Attraction.   [60- 

is  to  say,  when  it  experiences  no  resistance  from  the  air.  At  London 
this  is  32-19  feet. 

Since  the  velocity  which  a  force  imparts  to  a  movable  body  in  a 
given  time  is  greater  in  proportion  as  this  force  is  more  intense, 
the  force  of  gravity  in  different  places  is  measured  by  the  velocity 
which  it  imparts  to  a  body  falling  freely  in  a  vacuum  :  in  London, 
for  instance,  its  intensity  is  32-19  feet,  at  the  equator,  32-09,  and  at 
Spitzbergen  32*25  feet. 

61.  Application  of  the  pendulum  to  clocks. — The  regulation 
of  the  motion  of  clocks  is  effected  by  means  of  pendulums,  that  of 
watches  by  balance-springs.  Pendulums  were  first  applied  to  this 
purpose  by  Huyghens  in  1658,  and  in  the  same  year  Hooke  applied 
a  spiral  spring  to  the  balance  of  a  watch.  The  manner  of  employ- 
ing the  pendulum  is  shown  in  fig.  52.  The  pendulum  rod  passing 
between  the  prongs  of  a  fork,  a,  communicates  its  motion  to  a  rod,  b, 
which  oscillates  on  a  horizontal  axis,  o.  To  this  axis  is  fixed  a  piece, 
inn,  called  an  escapement  or  crutch,  terminated  by  two  projections 
or  pallets,  which  work  alternately  with  the  teeth  of  the  escapement 
wheel,  R.  This  wheel  being  acted  on  by  the  weight  tends  to  move 
continuously,  let  us  say,  in  the  direction  indicated  by  the  arrow- 
head. Now  if  the  pendulum  is  at  rest,  the  wheel  is  held  at  rest  by 
the  pallet,  M,  and  with  it  the  whole  of  the  clockwork  and  the  weight. 
If,  however,  the  pendulum  moves  and  takes  the  position  shown  by 
the  dotted  line,  m  is  raised,  the  wheel  escapes  from  the  confinement 
in  which  it  was  held  by  the  pallet,  the  weight  descends,  and  causes 
the  wheel  to  turn  until  its  motion  is  arrested  by  the  other  pallet,  ;/  ; 
which  in  consequence  of  the  motion  of  the  pendulum  will  be  brought 
into  contact  with  another  tooth  of  the  escapement  wheel.  In  this 
manner  the  descent  of  the  weight  is  alternately  permitted  and 
arrested — or,  in  a  word,  regulated — by  the  pendulum.  By  means 
of  a  proper  train  of  wheelwork  the  motion  of  the  escapement  is 
communicated  to  the  hands  of  the  clock  ;  and  consequently  their 
motion,  too,  is  regulated  by  the  pendulum. 

Hence,  to  regulate  a  clock  when  it  goes  too  slow  or  too  fast,  the 
length  of  the  pendulum  must  be  altered.  If  the  clock  goes  too 
slow,  it  is  because  the  pendulum  oscillates  too  slowly,  and  it  must 
therefore  be  shortened  ;  if,  on  the  contrary,  it  goes  too  fast,  it  must 
be  lengthened.  This  shortening  or  lengthening  is  usually  effected 
at  the  top  of  the  pendulum  by  varying  the  length  of  the  oscillating 
portion  of  the  plate  to  which  it  is  suspended.  Clocks  are  provided 


62] 


Metronome. 


55 


with  a  simple  arrangement  for  this  purpose,  which,  however,  is  not 
represented  in  the  figure. 

A  pendulum  which  makes  one  oscillation  in  a  second  is  called 
a  seconds  pendulum.  Its  length  is  not  the  same  in  different  parts 
of  the  earth  ;  it  is  somewhat  less  at  the  equator  than  at  the  poles. 
In  London  it  amounts  in  round  numbers  to  39*14  inches,  and  in 
New  York  to  39-10  inches. 

Seeing  that  heat  expands  bodies,  the  length  of  the  pendulum 
will  be  greater  in  summer,  and  less  in  winter.  Hence  a  clock  which 
has  been  once  regulated  for  the  mean  temperature,  will  lose  in 
summer  and  will  gain  in  winter.  How  this  effect  of  temperature  is 
counteracted  by  a  self-acting  arrangement,  will  be  seen  in  the 
chapter  on  Heat. 


Fig.  53- 

62.  Metronome. — This  is  another  application  of  the  isochronism 
of  the  oscillations  of  the  pendulum,  and  is  used  to  mark  the  time  in 


56    Properties  of  Matter  and  Universal  Attraction.  [62- 

practising  music.  As  this  time  varies  in  different  compositions,  it 
is  important  to  be  able  to  vary  the  duration  of  the  oscillations, 
which  is  effected  as  follows.  The  bob  of  the  pendulum,  B  (fig.  53). 
is  of  lead,  and  it  oscillates  about  an  axis,  o ;  the  rod  which  is  pro- 
longed above  this  axis  is  provided  with  a  weight,  A,  which  slides  on 
this  rod  and  can  be  fixed  in  any  position.  This  weight  obviously 
acts  in  opposition  to  the  oscillations  of  the  bob,  B,  for  when  this 
tends  to  oscillate,  for  instance,  from  right  to  left,  the  weight  tends 
to  move  the  rod  in  the  opposite  direction,  and  this  resistance  which 
it  affords  to  the  motion  is  greater  the  longer  the  arm  of  the  lever, 
A  0,  on  which  it  acts.  Hence  the  higher  the  weight,  A,  is  raised 
the  slower  are  the  oscillations.  At  the  base  of  the  instrument  there 
is  a  clockwork  motion,  which  works  an  escapement  with  such  force 
that,  at  each  oscillation  of  the  pendulum,  a  tooth  strikes  strongly 
against  a  pallet  fixed  to  the  axis,  o,  thus  producing  a  regular  beat 
which  gives  the  time.  In  front  of  the  box  which  contains  the 
mechanism  is  a  scale  with  numbers,  indicating  the  height  at  which 
the  weight  must  be  placed  to  obtain  a  given  number  of  oscillations 
in  a  minute.  In  the  drawing  this  weight  is  at  the  number  92, 
which  indicates  that  the  pendulum  makes  92  oscillations  in  a 
minute. 


CHAPTER   VI. 

MOLECULAR   ATTRACTION. 

63.  Cohesion  and  chemical  affinity. — After  having  described, 
under  the  name  of  universal  gravitation,  the  attraction  which 
exists  between  the  stars  and  planetary  bodies  ;  and  under  that  of 
gravity,  the  attraction  which  the  earth  exerts  upon  all  bodies  in 
making  them  fall  towards  it,  we  have  to  investigate  the  attractions 
which  hold  together  the  ultimate  particles  or  molecules  of  a  body. 
These  are — cohesion,  affinity,  and  adhesion. 

Cohesion  is  the  force  which  unites  two  molecules  of  the  same 
nature  ;  for  example,  two  molecules  of  water,  or  two  molecules  of 
iron.  Cohesion  is  strongly  exerted  in  solids,  less  strongly  in  liquids, 
and  scarcely  at  all  in  gases.  Its  intensity  decreases  as  the  tem- 
perature increases,  because  then  the  repulsive  force  due  to  heat 
increases.  Hence  it  is,  that  when  solid  bodies  are  heated,  they  first 


-63]  Molecular  Attraction.  57 

expand,  then  liquefy,  and  are  ultimately  converted  into  the  gaseous 
state,  provided  that  heat  produces  in  them  no  chemical  change. 

Cohesion  varies  not  only  with  the  nature  of  bodies,  but  also  with 
the  arrangement  of  their  molecules  ;  for  example,  the  difference 
between  tempered  and  untempered  steel  is  due  to  a  difference  in 
the  molecular  arrangement  produced  by  tempering.  Many  of  the 
properties  of  bodies,  such  as  tenacity,  hardness,  and  ductility,  are 
due  to  the  modifications  which  this  force  undergoes. 

In  large  masses  of  liquids,  the  force  of  gravity  overcomes  that  of 
cohesion.  Hence  liquids  acted  upon  by  the  former  force  have  no 
special  shape  ;  they  take  that  of  the  vessel  in  which  they  are  con- 
tained. But  in  smaller  masses  cohesion  gets  the  upper  hand,  and 
liquids  present  then  the  spheroidal  form.  This  is  seen  in  the  drops 
of  dew  on  the  leaves  of  plants  ;  it  is  also  seen  when  a  liquid  is 
placed  on  a  solid  which  it  does  not  moisten  ;  as,  for  example, 
mercury  upon  wood.  The  experiment  may  also  be  made  with 
water,  by  sprinkling  upon  the  surface  of  the  wood  some  light 
powder  such  as  lycopodium  or  lampblack,  and  then  dropping  some 
water  on  it. 

Chemical  affinity  is  the  force  which  is  exerted  between  molecules 
not  of  the  same  kind.  Thus,  in  water,,  which  is  composed  of 
oxygen  and  hydrogen,  it  is  affinity  which  unites  these  elements,  but 
it  is  cohesion  which  binds  together  two  molecules  of  water.  In 
compound  bodies  cohesion  and  affinity  operate  simultaneously, 
while  in  simple  bodies  cohesion  has  alone  to  be  considered. 

To  affinity  are  due  all  the  phenomena  of  combustion  ;  when 
carbon  burns  it  is  affinity  which  causes  this  body  to  combine  with  the 
oxygen  of  the  air  to  form  the  gas  known  as  carbonic  acid.  Affinity 
determines  the  combination  of  the  elements,  so  that  with  a  small 
number  of  them  are  formed  the  immense  number  of  organic  and 
mineral  substances  which  serve  for  our  daily  uses. 

The  causes  which  tend  to  weaken  cohesion  are  most  favourable 
to  affinity  ;  for  instance,  the  action  of  affinity  between  substances 
is  facilitated  by  their  division,  and  still  more  by  converting  them  to 
u  liquid  or  gaseous  state.  It  is  most  powenully  exerted  by  a  body 
in  its  nascent  state,  that  is,  the  state  in  which  the  body  exists  at  the 
moment  it  is  disengaged  from  a  compound  ;  the  body  is  then  free, 
and  ready  to  obey  the  feeblest  affinity.  An  increase  of  temperature 
modifies  affinity  differently  under  different  circumstances.  In  some 
cases,  by  diminishing  cohesion,  and  increasing  the  distance  between 
the  molecules,  heat  promotes  combination.  Sulphur  and  oxygen, 


58    Properties  of  Matter  and  Universal  Attraction.    [63- 

which  at  the  ordinary  temperature  are  without  action  on  each 
other,  combine  to  form  sulphurous  acid  when  the  temperature  is 
raised.  In  other  cases  heat  tends  to  decompose  compounds  ;  thus 
many  metallic  oxides,  as  for  example  those  of  silver  and  mercury, 
are  decomposed,  by  the  action  of  heat,  into  gas  and  metal. 

64.  Adhesion. — Adhesion  is  the  name  given  to  the  attraction 
manifested  by  two  bodies  when  their  surfaces  are  placed  in  con- 
tact.    If  two  leaden  bullets  are  cut  with  a  penknife  so  as  to  form 
two  equal  and  brightly   polished  surfaces,  and  the  two  faces  are 
turned  against  each  other  until  they  are  in  the  closest  contact,  they 
adhere  so  strongly  as  to  require  a  force  of  more  than  3   or  4  ounces 
to  separate  them.     The  same  experiment  may  be  made  with  two 
equal  pieces  of  glass,  which  are  polished  and  made  perfectly  plane. 
When  they  are  pressed  one  against  the  other,  the  adhesion  is  so 
powerful  that  they  cannot  be  separated  without  breaking.     As  the 
experiment  succeeds  in  vacuo,  it  cannot  be  due   to  atmospheric 
pressure,  but  must  be  attributed  to  a  reciprocal  action  between  the 
two  surfaces.     The  attraction  also  increases  as  the  contact  is  pro- 
longed, and  is  greater  in  proportion  as  the  contact  is  closer. 

To  adhesion  is  due  the  resistance  experienced  in  raising  a  plank 
placed  on  water  ;  and  to  the  same  force  is  ascribed  the  difficulty 
met  with  in  walking  through  thick  mud.  If  we  dip  a  glass  rod  into 
water,  on  withdrawing  it  a  drop  will  be  found  to  collect  at  the 
bottom,  and  remain  suspended  there.  As  the  weight  of  the  drop 
tends  to  detach  it,  there  must  necessarily  be  some  force  superior 
to  this  weight  which  maintains  it  there  ;  this  force  is  the  force  of 
adhesion.  On  the  property  of  adhesion,  depends  the  operations  of 
gluing  and  soldering,  of  cementing,  and  coating  mirrors. 

The  force  of  adhesion  operates  also  between  solids  and  gases. 
If  a  metal  plate  be  immersed  in  water  bubbles  will  be  found,  to 
appear  on  the  surface.  As  air  cannot  penetrate  into  the  pores  of  the 
plate,  the  bubbles  could  not  arise  from  air  which  had  been  expelled, 
but  must  be  due  to  a  layer  of  air  which  covered  the  plate  and 
moistened  it  like  a  liquid. 

CAPILLARITY.      ABSORPTION. 

65.  Capillary  phenomena. — When  solid  bodies  are  placed  in 
contact  with  liquids,  molecular  attraction  gives  rise  to  a  class  of 
phenomena  called  capillary  phenomena,  because  they  are  best  seen 
in  tubes  whose  diameters  are  comparable  with  the  diameter  of  a 
hais.     These  phenomena  are  treated  of  in  physics  under  the  head 


-66] 


Capillarity.     A  bsorption. 


59 


of  capillarity  or  capillary  attraction  :  the  latter  expression  is  also 
applied  to  the  force  which  produces  the  phenomena. 

The  phenomena  of  capillarity  are  very  various,  but  may  all  be 
referred  to  the  mutual  attraction  of  the  liquid  molecules  for  each 
other,  and  to  the  attraction  between  these  molecules  and  solid 
bodies.  The  following  are  some  of  these  phenomena  : — 

i.  When  a  glass  rod  is  placed  in  a  liquid  which  wets  it,  water  for 
instance,  the  liquid,  as  if  not  subject  to  the  laws  of  gravity,  is  raised 
upwards  against  the  sides  of  the  solid,  and  its  surface,  instead  of 
being  horizontal,  becomes  slightly  concave  (fig.  54). 


Fig-  54-  Fig   55-  Fig.  56. 

ii.  If  instead  of  a  solid  rod,  a  hollow  tube  be  immersed  in  water 
(fig-  55)>  n°t  merely  is  the  liquid  raised  around  the  tube,  but  it  rises 
in  the  inside  to  a  height  which  is  greater,  the  narrower  the  tube  ; 
and  at  the  same  time  the  surface  of  the  liquid  inside  the  tube 
assumes  a  concave  form. 

iii.  If  the  tube  is  not  moistened  by  the  liquid,  as  is  the  case  with 
mercury,  the  liquid  is  depressed  instead  of  being  raised,  and  the 
more  so  the  narrower  the  tubes  (fig.  56)  ;  and  the  surface,  which 
was  previously  concave,  now  becomes  convex.  The  surface  of  a 
liquid  exhibits  the  same  concavity  or  convexity  against  the  sides  of 
a  vessel  in  which  it  is  contained,  according  as  the  sides  are  or  are 
not  moistened  by  the  liquid. 

66.  Xiaws  of  capillarity. — Gay-Lussac  has  shown  experimentally 
that  the  elevation  and  depression  of  liquids  in  capillary  tubes,  the 
internal  diameter  of  which  does  not  exceed  two  millimetres,  are 
governed  by  the  following  laws  : — 

I.  When  a  capillary  tube  is  placed  in  a  liquid,  the  liquid  is 
raised  or  depressed  according  as  it  does  or  does  not  moisten  the 


60    Properties  of  Matter  and  Universal  .A  t  tract  ion.  [66  - 

tube,  and  the  elevation  -varies  inversely  as  the  diameter  of  the  tube, 
that  is,  it  is  two  or  three  times  as  great  when  this  diameter  is  two 
or  three  times  as  small. 

II.  The  elevation  varies  -with  the  nature  of  the  liquid,  and  ivith 
the  temperature,  but  is  independent  of  the  nature  and  thickness  oj 
the  tube. 

67.  Effects  due  to  capillarity. — It  is  from  capillarity  that  sap 
rises  in  plants,  that  oil  rises  in  the  wicks  of  lamps,  and  melted 
tallow  in  the  wicks  of  candles.     The  interstices  which  exist  between 
the  fibres  of  the  cotton  of  which  the  wicks  are  formed,  act  as  capillary 
tubes  in  which  the  ascent  takes  place.      In  very  porous  bodies,  the 
pores  being  in  communication  with   each  other  form  a  series  of 
capillary  tubes,  which  produces  the   same   effect.      If  a  lump  of 
sugar  be  placed  in  a  cup  in  which  a  little  coffee  is  left,  the  liquid 
is  seen  to  rise  rapidly  and  fill  the  entire  piece  ;  and  it  is  even  to  be 
remarked  that  the  sugar  then  dissolves  more  quickly  than  if  it  had 
been  directly  immersed  in  the  coffee.     This  is  due  to  the  fact  that 
in  the  latter  case  the  air  which  fills  the  pores  not  being  able  to 
escape  so  rapidly,  as  if  the  piece  of  sugar  is  only  partially  immersed, 
prevents  the  liquid  from  penetrating  into  the  mass  of  the  sugar,  and 
thus  retards  the  solution. 

Insects  can  often  move  on  the  surface  of  water  without 
sinking.  This  is  a  capillary  phenomenon  caused  by  the  fact,  that 
as  their  feet  are  not  wetted  by  the  water,  a  depression  is  produced 
which  keeps  them  up  in  spite  of  their  weight.  Similarly  a  sewing 
needle  gently  placed  on  water  does  not  sink,  because  its  surface, 
being  covered  with  an  oily  layer,  does  not  become  wetted.  But  if 
previously  washed  in  alcohol,  or  in  potash,  it  at  once  sinks  to  the 
bottom. 

68.  Absorption  and  imbibition. — The  words  absorption  and 
imbibition  are  used  almost  promiscuously  in  physics  ;  they  indicate 
the  penetration  of  a  liquid  or  a  gas  into  a  porous  body.      Absorp- 
tion is  used  both  for  liquids  and  gases,  while  imbibition  is  restricted 
to  liquids. 

Charcoal  has  a  great  absorbing  power  for  gases.  If  a  piece  of 
recently  heated  charcoal  be  passed  into  a  bell  jar  full  of  carbonic 
acid  placed  over  a  mercury  trough,  the  volume  of  gas  is  seen  to 
diminish  rapidly,  and  it  is  found  that  the  gas  which  has  disap- 
peared, in  penetrating  the  charcoal  represents  a  volume  thirty-five 
times  that  of  the  solid.  There  are  even  gases,  such  as  ammonia, 
of  which  charcoal  can  absorb  ninety  times  its  own  volume. 


-69]  Effects  due  to  Imbibition.  61 

Absorption  takes  place  in  all  parts  of  plants,  but  more  especially 
in  the  rootlets  and  by  the  leaves.  These  organs  absorb,  in  the 
form  of  water,  carbonic  acid,  and  ammonia,  the  oxygen,  hydrogen, 
carbon,  and  nitrogen  necessary  for  the  growth  of  the  plants. 

Absorption  also  plays  an  important  part  both  in  the  nutrition 
and  respiration  of  animals.  Animal  tissues  can  even  absorb  solid 
substances.  For  instance,  in  those  processes  of  the  arts  where  the 
workmen  have  to  handle  salts  of  mercury  or  of  lead,  these  metals 
are  gradually  absorbed  into  the  system  and  produce  serious  evils. 

69.  effects  due  to  imbibition. — Imbibition  has  been  defined 
as  being  the  penetration  of  a  liquid  into  the  pores  of  a  solid  body. 
It  is  a  capillary  effect,  for  the  pores  being  in  intercommunication 
act  like  small  tubes  ;  thus  it  is  that  water  rises  in  wood,  sponge, 
bibulous  paper,  sugar,  sand,  and  in  all  bodies  which  possess  pores 
of  a  perceptible  size. 

Owing  to  imbibition,  tobacco  soon  dries  if  kept  in  a  wooden 
box,  while  it  remains  fresh  if  kept  in  a  metal  one,  for  then  its 
moisture  is  not  absorbed  by  the  metal  as  it  is  by  the  wood. 

When  water  is  absorbed  by  animal  or  vegetable  matters  their 
volume  increases.  Thus  if  a  tolerably  large  sheet  of  dry  paper  be 
measured  and  be  then  moistened,  it  will  be  found  to  have  appreciably 
increased  by  this  process.  This  property  is  made  use  of  in 
stretching  paper  on  drawing  boards  ;  the  paper  is  moistened  and 
is  then  glued  or  fastened  with  pins  round  the  edge  of  the  board. 
In  drying,  the  paper  contracts,  and  is  tightly  stretched.  For  the 
same  reason,  too,  wall  papers  which  have  been  fastened  on  cloth 
along  the  walls,  are  frequently  liable  to  be  torn. 

In  bending  wood,  the  side  to  be  bent  is  heated,  and  the  other 
side  moistened.  This  being  lengthened  owing  to  the  water  it 
absorbs,  while  the  other  is  contracted  in  consequence  of  the  dry- 
ness,  a  curvature  ensues  on  the  heated  side. 

It  is  often  observed  that,  owing  to  the  changes  of  volume 
which  they  undergo  under  the  influence  of  moisture  and  dryness, 
the  furniture  of  our  rooms  is  frequently  heard  to  crack  when  the 
weather  changes. 

By  the  absorption  of  moisture  ropes  become  shorter  ;  and 
lengthen  when  they  dry.  This  may  seem  opposed  to  what  has 
been  stated  about  moistened  paper,  but  the  explanation  is  not 
difficult.  Ropes  are  formed  of  fibres  twisted  together,  and  as 
these  fibres  swell  owing  to  the  water  they  absorb,  the  rope  becomes 
larger,  and  hence  each  fibre  should  make  in  coiling  a  longer 


62    Properties  of  Matter  and  Universal  Attraction.  [69- 

circuit ;  and  the  rope  will  become  more  shortened  the  more  it  is 
moistened.  For  this  reason,  too,  new  cloths  shrink  considerably 
when  they  are  moistened  for  the  first  time. 

It  is  related  that  Pope  Sixtus,  wishing  to  raise  in  a  place  in  Rome, 
an  obelisk  brought  from  Heliopolis  to  Rome  under  Caligula,  for 
fear  of  disturbing  the  operation,  ordered  the  spectators  to  preserve 
profound  silence  under  pain  of  death.  The  obelisk  was  on  the 
point  of  being  placed  on  its  pedestal,  when  the  ropes  began  to 
stretch,  owing  to  the  great  traction  to  which  they  were  exposed, 
and  the  operation  was  in  great  danger.  A  voice  from  the  crowd — 
that  of  the  architect  Zapaglia — cried  out,  '  Wet  the  ropes/  which 
was  done,  and  the  operation  successfully  performed. 


CHAPTER  VII. 

PROPERTIES   SPECIAL  TO   SOLIDS. 

70.  Tenacity. — Besides  the  general  properties  which  we  have 
hitherto  been  considering,  and  which  are  met  with  in  solids, 
liquids,  and  gases,  there  are  some  special  to  solids  which  deserve 
mention,  on  account  of  the  numerous  applications  which  they 
present.  They  are — tenacity,  hardness,  ductility,  and  malleability. 

Tenacity  is  the  resistance  which  bodies  oppose  to  being  broken, 
when  subjected  to  a  greater  or  less  traction.  The  tenacity  of  any 
particular  body  is  determined  by  giving  to  it  the  form  of  a  cylin- 
drical or  prismatic  rod,  one  end  of  which  is  then  firmly  fixed  in 
a  vertical  position  to  a  support.  To  the  lower  end  is  fixed  a  scale- 
pan,  in  which  weights  are  successively  added  until  the  rod  breaks. 
The  breaking  weight  represents  the  limit  of  the  tenacity  of  a  rod 
for  a  given  section. 

Of  all  substances  iron  has  the  greatest  tenacity.  A  cylindrical 
iron  rod  with  a  section  of  a  square  centimetre,  only  breaks  with  a 
weight  of  13,200  pounds.  A  rod  of  boxwood  of  the  same  dimensions, 
breaks  with  a  weight  of  2,640,  and  one  of  oak  with  1,540  pounds  ; 
a  steel  wire  supports  a  load  of  39,000  times  its  own  length  ;  laths 
constructed  of  fine  iron  wire,  the  ^tn  to  ^th  of  an  inch  in  diameter, 
can  support  a  load  of  60  tons  for  each  square  inch  of  section. 

Tenacity  is  directly  proportional  to  the  breaking  weight,  and 
inversely  proportional  to  the  area  of  a  transverse  section  of  the  wire. 

Tenacity  diminishes  with  the  duration  of  the  traction.     A  small 


-72]  Tenacity.  63 

force  continuously  applied  for  a  long  time  will  often  break  a  wire, 
which  would  not  at  once  be  broken  by  a  larger  weight. 

Not  only  does  tenacity  vary  with  different  substances,  but  it  also 
varies  with  the  form  of  the  body.  Thus,  with  the  same  sectional 
area,  a  cylinder  has  greater  tenacity  than  a  prism.  The  quantity 
of  matter  being  the  same,  a  hollow  cylinder  has  greater  tenacity 
than  a  solid  one. 

The  shape  has  also  the  same  influence  on  the  resistance  to 
crushing,  as  it  has  on  the  resistance  to  traction.  A  hollow  cylinder 
with  the  same  mass,  and  the  same  weight,  offers  a  greater  resistance 
than  the  solid  cylinder.  It  is  for  this  reason  that  the  bones  of 
animals,  the  feathers  of  birds,  the  stems  of  corn  and  other  plants, 
offer  greater  resistance  than  if  they  were  solid,  the  mass  remaining 
the  same. 

71.  Hardness. — Hardness  is  the  resistance  which  bodies  offer  to 
being  scratched  or  worn  by  others.      It  is  only  a  relative  property, 
for  a  body  which  is  hard  in  reference  to  one  body,  may  be  soft  in 
reference  to  others.      The  relative  hardness  of  two  bodies  is  ascer- 
tained by  trying  which  of  them  will  scratch  the  other.     Diamond  is   jjfl 
the  hardest  of  all  bodies,  for  it  scratches  all,  and  is  not  scratched  by 
any.     The  hardness  of  a  body  is  expressed  by  referring  it  to  a  scale  * 
of  hardness  :  that  usually  adopted  is — 

1.  Talc  5.  Apatite  8.  Topaz 

2.  Rock  salt  6.  Felspar  9.  Corundum 

3.  Calcspar  7.  Quartz  10.  Diamond 

4.  Flourspar 

Thus  the  hardness  of  a  body  which  would  scratch  felspar,  but  would 
be  scratched  by  quartz,  would  be  expressed  by  the  number  6-5. 

The  pure  metals  are  softer  than  their  alloys.  Hence,  for 
jewellery  and  coinage,  gold  and  silver,  which  are  soft  metals,  are 
alloyed  with  copper  to  increase  their  hardness. 

The  hardness  of  a  body  has  no  relation  to  its  resistance  to  com- 
pression. Glass  and  diamond  are  much  harder  than  wood,  but  the 
latter  offers  far  greater  resistance  to  the  blow  of  a  hammer.  Hard 
bodies  are  often  used  for  polishing  powders  ;  for  example,  emery, 
pumice,  and  tripoli.  Diamond,  being  the  hardest  of  all  bodies,  can 
only  be  ground  by  means  of  its  own  powder. 

72.  Ductility. — Ductility  is  the  property  in  virtue  of  which  a 
great  number  of  bodies  change  their  forms  by  the  action  of  traction 
or  pressure. 


64    Properties  of  Matter  and  Universal  A  ttraction.   [72- 

Certain  bodies,  such  as  clay,  wax,  etc.,  are  so  ductile  that  they 
can  be  drawn  out,  flattened,  modelled,  between  the  fingers  ;  others, 
such  as  the  resins  and  glass,  require  the  aid  of  heat.  Glass  is  then 
so  ductile  that  it  can  be  drawn  out  into  fine  threads,  which  are 
flexible  enough  to  be  woven  into  cloth. 

Several  metals,  such  as  gold,  silver,  copper,  are  ductile,  even  at 
ordinary  temperatures,  but  require  the  use  of  powerful  machines, 
such  as  the  draw-plate  or  the  rolling-mill. 

73.  Malleability. — Malleability  is  that  modification  of  ductility 
which  is  exhibited  when  metals  are  hammered.  This  property 
greatly  increases  with  the  temperature  ;  everyone  knows,  for  in- 
stance, that  iron  is  easily  forged  when  hot,  and  not  when  cold. 

Gold  is  very  malleable  even  at  the  ordinary  temperature.  To 
make  the  extremely  thin  plates  of  gold,  known  as  gold  leaf,  the  gold 
is  first  pressed,  by  means  of  the  rolling  mill  into  long  plates  from 
two  to  three  centimetres  in  breadth,  and  about  a  millimetre,  the  ^th 
of  an  inch  in  thickness.  These  plates  are  then  beaten  into  small 
squares  by  means  of  a  hammer ;  these  are  then  cut  and  beaten  again, 
and  so  on.  By  beating  them  directly,  the  operation  could  not  long 
be  continued,  for  the  metal  would  be  torn  :  hence  the  plates  to  be 
beaten  must  be  placed  between  plates  of  a  substance  which,  while 
thin,  affords  great  resistance.  Sheets  of  vellum  and  parchment  are 
first  used  for  this  purpose,  and  afterwards  gold  beater's  skin. 

Leaves  of  gold  are  thus  obtained,  which  are  so  thin,  that  20,000 
superposed  are  only  an  inch  thick.  Silver  and  copper  may  also  be 
worked  in  the  same  manner.  These  leaves  are  used  in  the  arts  for 
gilding  on  wood,  paper,  and  other  materials. 

The  following  is  the  usual  order  of  the  metals  under  the  draw- 
plate,  the  rolling  mill,  and  the  hammer,  arranged  in  reference  to 
their  decreasing  ductility. 

Draw-plate  Rolling  mill  Hammer 

Platinum  Gold  Lead 

Silver  Silver  Tin 

Iron  Copper  Gold 

Copper  Tin  Zinc 

Gold  Lead  Silver 

Zinc  Zinc  Copper 

Tin  Platinum  i  Platinum 

Lead  Iron  Iron 

The  metals  must  be  pure,  if  they  are  alloyed  with  other  metals 
they  are  fragile,  and  have  but  little  ductility. 


-75]  Special  Characteristics  of  Liquids.  6$ 


BOOK  II. 

HYDROSTATICS. 

CHAPTER   I. 
PRESSURES  TRANSMITTED  AND   EXERTED    BY  LIQUIDS. 

74.  Province  of  Hydrostatics. — The  science  of  hydrostatics, 
from  two  Greek  words,  signifying  equilibrium  of  water,  treats  of 
the  conditions  of  the  equilibrium  of  liquids,  and  of  the  pressure 
they  exert,  whether  within  their  own  mass,  or  on  the  sides  of  the 
vessels  in  which  they  are  contained. 

75.  Special  characteristics  of  liquids. — One  essential  charac- 
ter of  a  liquid  is  the  extreme  mobility  of  its  molecules,  which  are 
displaced  by  the  slightest  force.     The  fluidity  of  liquids  is  due  to 
this  property  ;  it,  however,  is  not  perfect,  there  is  always  a  sufficient 
adherence  between  the  molecules   to   produce   a  greater  or  less 
viscosity. 

Another  essential  property  of  liquids,  and  one  by  which  they  are 
distinguished  from  gases,  is  their  almost  entire  incompressibility. 
We  have  already  seen  (5)  that  their  compressibility  is  so  small,  that 
for  a  long  time  they  were  regarded  as  being  quite  incompressible. 
It  was  not  before  1823  that  Oersted,  a  Swedish  physicist,  first  proved 
in  an  exact  manner  that  liquids  are  compressible.  The  apparatus 
he  used  for  this  purpose  is  called  the  piezometer  (7rif£w,  I  compress, 
/itrpor,  measure).  By  its  means  it  has  been  found  that  a  pressure  of 
one  atmosphere  compresses  distilled  water  by  about  the  ^~  part 
of  its  volume  ;  mercury  by  the  same  pressure  only  undergoes  about 
a  tenth  as  great  a  diminution,  and  ether  about  2^  times  as  much. 

Liquids  are  also  porous,  elastic,  and  impenetrable,  like  all  other 

F 


66 


Hydrostatics. 


[75- 


bodies.  The  proofs  of  their  porosity  have  been  already  given, 
their  elasticity  is  a  necessary  consequence  of  their  compressibility. 
Their  impenetrability  is  manifested  whenever  a  solid  is  immersed 
in  water.  For  if  a  vessel  be  quite  filled  with  water,  and  any  solid 
body  be  placed  in  it  which  does  not  absorb  the  liquid,  it  will  be 
observed  that  a  volume  of  water  flows  over,  which  is  exactly  equal 
to  that  of  the  solid  immersed. 

76.  Equality  of  pressures.  Pascal's  law. — Liquids  have  the 
following  remarkable  property,  which  is  not  possessed  by  solids.  It 
is  often  called  'Pascal's  law/  for  it  was  first  enunciated  by  that  dis- 
tinguished geometrician. 

Pressure  exerted  anywhere  iipon  a  mass  of  liquid  is  transmitted 
undiminished  in  all  directions,  and  acts  with  the  same  force  on  all 
equal  surfaces,  and  in  'a  direction  at  right  angles  to  those  sur- 
faces. 

To  get  a  clearer  idea  of  the  truth  of  this  principle,  let  us  conceive 
a  cylindrical  vessel,  in  the  sides  of  which  are  placed  various  cylin- 
drical tubulures,  all  of  the  same 
size,  and  closed  by  movable 
pistons  (fig.  57).  The  vessel 
being  filled  with  water,  or  any 
other  liquid,  the  moment  any 
pressure  is  applied  to  the  piston 
A,  all  the  other  pistons  are 
pressed  outwards,  showing  that 
the  pressure  is  not  merely  trans- 
mitted downwards  upon  the 
piston  D,  but  laterally  upon  the 
pistons  E  and  F,  and  upwards 
upon  the  pistons  B  and  C.  If, 
instead  of  pressing  on  the  piston 
A,  the  pressure  be  exerted  upon 
B,  the  same  effects  are  produced ;  the  piston  A  is  then  forced 
upwards. 

In  these  different  cases,  not  only  is  the  pressure  transmitted  in  all 
directions,  but  for  the  same  surface  it  is  transmitted  with  the  same 
intensity.  For  instance,  if  the  pressure  on  the  piston  A  is  twenty 
pounds,  and  its  surface  is  equal  to  that  of  the  piston  B,  the  upward 
pressure  on  the  latter  is  also  twenty  pounds  ;  but  if  the  surface  of 
the  piston  B  is  only  a  twentieth  that  of  A,  the  pressure  upon  B  is 
only  one  pound.  This  is  the  principle  of  the  eqiiality  of  pressure. 


Fig.  57- 


-78]      Pressures  resulting  from  Weight  of  Liquids.      67 


Fig.  58. 


77.  Consequence  and  verification  of  Pascal's  principle. — It 

follows  from  what  has  been  said,  that  the  pressure  transmitted  by  a 
liquid  is  proportional  to  the 
extent  of  surface  ;  this  is  in- 
deed only  another  enuncia- 
tion of  Pascal's  principle.  . 

To  verify  this,  two  cylin- 
ders are  taken  of  unequal  di- 
mensions, joined  by  a  tube 
(fig.  58).  These  cylinders 
contain  water,  and  are  pro- 
vided with  pistons  which 
move  in  them  with  gentle  friction.  Now  if  the  surface  of  the  larger 
one,  P,  for  instance,  is  twenty  times  that  of  the  smaller  one,  /,  it 
will  be  found  that  a  weight  of  a  pound  placed  upon  p  will  balance 
a  weight  of  twenty  pounds  placed  upon  P  ;  if  these  weights  are  in 
any  other  ratio,  equilibrium  is  destroyed. 

The  principle  of  the  equality  of  pressures  forms  the  basis  of  the 
whole  science  of  hydrostatics,  and  we  shall  presently  find  a  very 
important  application  of  it  in  the  hydraulic  press  (83). 

78.  Pressures    resulting:   from    the    weight    of    liquids. — In 
what  has  been  said,  we  have  considered  the  pressures  transmitted 
towards  the  sides  of  the  vessel,  when  some  external  force  is  applied. 
It  is  not,  however,  necessary  to  exert  an  external  pressure  on  the 
surface  of  a  liquid  in  order  to  produce  internal  pressures  in  its  mass, 
and  on  the   sides   of  the  vessel.     The  mere  weight  of  the  liquid 
itself  is  sufficient  to  produce  pressures  which  vary  with  the  depth 
and  with  the  density  of  the  liquid. 

For  suppose  any  vessel  filled  with  liquid  ;  if  we  conceive  the 
liquid  divided  into  horizontal  layers  of  equal  thickness,  it  is  clear 
that  the  second  layer  supports  a  pressure  equal  to  the  weight  of  the 
first  ;  that  the  third  supports  the  weight  of  the  first  and  second,  and 
so  on ;  so  that  the  pressure  increases  with  the  number  of  layers, 
which  is  expressed  by  saying  that  gravity  produces  in  liquids  pres- 
sures proportional  to  the  depth. 

It  is  obvious,  moreover,  that  these  pressures  are  proportional  to 
the  density  of  the  liquids  ;  that  is,  that  for  the  same  depth,  a  liquid 
which  has  two  or  three  times  the  density  of  another,  will  exert  twice 
or  thrice  as  much  pressure. 

It  follows  from  the  principle  of  the  equality  of  pressure  in  all 
directions,  that  the  pressure  produced  by  gravity  in  liquids  is  exerted 

F  2 


68 


Hydrostatics 


[78- 


not  merely  in  the  direction  of  this  force,  but  horizontally,  and  also 
upwards,  as  will  now  be  demonstrated. 

79.  lateral  pressures.    Hydraulic  tourniquet. — The  existence 
of  lateral  pressures  which  liquids  exert  upon  the  sides  of  the  vessel 

in  which  they  are  contained, 
may  be  demonstrated  by 
means  of  the  hydraulic 
tourniquet  or  Barker's  mill 
(fig.  59).  This  consists  essen- 
tially of  a  long  glass  tube,  C, 
with  a  funnel,  D,  at  the 
top.  The  bottom  of  the  tube 
fits  into  a  hollow  brass 
box,  which  rests  on  a  pivot ; 
in  the  sides  of  the  box  are 
fitted  four  brass  tubes,  ar- 
ranged crosswise,  and  all 
bent  in  the  same  direction  at 
the  ends. 

Waterdescendingthe  long 
tube  emerges  by  the  aper- 
tures of  the  bent  tubes,  which 
are  soon  seen  to  rotate  rapidly 
in  the  direction  indicated  by 
the  arrow.  This  rotation  is 
due  to  the  lateral  pressure 
exerted  by  the  column  of 
water  in  the  long  tube .  For 
let  us  consider  one  of  the 
bent  tubes,  «A,  B£,  repre- 

____  sented  in  section  on  the  left 

(fig.  59),  and  suppose  first 
that  the  orifices,  a  and  b,  are 

•  closed.  The  column  of  water  which  then  fills  the  tube  C  exerts 
upon  the  portions  of  the  opposite  sides,  A  and  a,  equal  and  contrary 
pressures  which  hold  each  other  in  equilibrium ;  this  is  also  the  case 
at  B  and  b,  and  thus  no  rotation  can  be  produced  in  either  direction. 
But  if  the  orifices  a  and  b  are  open,  as  is  the  case  when  the  appa- 
ratus is  at  work,  as  the  water  issues  by  these  orifices,  the  pressures 
at  a  and  b  no  longer  exist ;  while  those  transmitted  to  A  and  B 
continuing  to  act,  produce  the  rotation. 


-81]       Pressure  is  Independent  of  Form  of  Vessel.        69 


Rotating  fireworks  also  act  on  the  same  principle  as  Barker's 
mill ;  that  is,  an  unbalanced  reaction  from  the  heated  gases  which 
issue  from  openings  in  them  gives  them  motion  in  the  opposite 
directions. 

It  is  in  consequence  of  the  lateral  pressure  of  water  that  dykes 
and  banks  which  retain  rivers  or  reservoirs,  sometimes  give  way, 
by  becoming  too  weak  for  the  pressure  they  have  to  support. 

80.  vertical  upward  pressure. — The  pressure  which  the  upper 
layers  of  a  liquid  exert  on  the  lower  layers  causes  them  to  exert  an 
equal  reaction  in  an  upward  direction,  a  necessary  consequence  qf 
the  principle  of  transmission  of  pressure  in  all  directions. 

The  following  experiment  (fig.  60)  serves  to  exhibit  the  upward 
pressure  of  liquids.  A  large  open 
glass  tube,  one  end  of  which  is 
ground,  is  fitted  with  a  ground 
glass  disc,  a,  or  still  better,  with  a 
thin  card  or  piece  of  mica,  the 
weight  of  which  may  be  neglected. 
To  the  disc  is  fitted  a  -string,  b,  by 
which  it  can  be  held  against  the 
bottom  of  the  tube.  The  whole 
is  then  immersed  in  water,  and 
the  disc  does  not  fall,  although  no 
longer  held  by  the  string  ;  it  is 
consequently  kept  in  its  position 
by  the  upward  pressure  of  the 
water.  If  water  be  now  slowly 
poured  into  the  tube,  the  disc  will  Fis-  6o- 

only  sink  when  the  height  of  the  water  inside  the  tube  is  equal  to 
the  height  outside.  It  follows  thence  that  the  upward  pressure  on 
the  disc  is  equal  to  the  pressure  of  a  column  of  .water,  the  base  of 
which  is  the  internal  section  of  the  tube  «,  and  the  height  the  dis- 
tance from  the  disc  to  the  outer  surface  of  the  liquid.  Hence  the 
iipward  pressure  of  liquids  at  any  point  is  governed  by  the  same 
laws  as  the  downward  pressure. 

This  upward  pressure  is  termed  the  buoyancy  of  liquids ;  it  is 
perceived  when  the  hand  is  plunged  into  water,  and  still  more  dis- 
tinctly if  it  is  immersed  in  mercury,  which  being  of  greater  density 
produces  greater  pressure.  It  is  owing  to  this  buoyancy  that,  if  a 
hole  be  made  in  the  bottom  of  a  ship,  water  enters  with  force. 

8 1.  Pressure  is  independent  of  the  shape  of  the  vessel. — The 


Hydrostatics. 


[81- 


pressure  exerted  by  a  liquid,  in  virtue  of  its  weight,  on  any  portion 
of  the  liquid,  or  on  the  sides  of  the  vessel  in  which  it  is  contained, 
depends  on  the  depth  and  density  of  the  liquid,  but  is  independent 
of  the  shape  of  vessel  and  of  the  quantity  of  the  liquid. 

This  principle,  which  follows  from  the  law  of  the  equality  of 
pressure,  may  be  experimentally  demonstrated  by  many  forms  of 
apparatus.  The  following  is  one  frequently  used,  and  is  due  to 
Masson.  It  consists  of'  a  large  conical  vessel,  M,  screwed  to 
a  brass  tubulure,  r,  fixed  to  a  wooden  support  (fig.  61).  This 
tubulure  is  closed  by  a  disc,  a,  which  does  not  adhere  to  it,  but  is 


Fig.  61. 

simply  applied  against  the  edge,  and  is  kept  there  by  a  string 
attached  to  one  end  of  an  ordinary  balance,  to  the  other  end  of 
which  is  a  scale-pan.  Weights  are  placed  in  the  latter,  so  as  just 
to  counterbalance  the  pressure  of  the  water  on  the  disc,  when  the 
vessel  M  is  almost  full ;  water  is  then  gradually  added  until  the 
disc  just  begins  to  give  way  and  allows  some  to  escape.  A  rod, 
o,  is  then  lowered  until  its  point  just  grazes  the  surface  of  the 
liquid.  If  the  vessel  M  be  unscrewed  and  replaced  by  the  cy- 
lindrical tube,  P,  the  capacity  of  which  is  far  less,  on  gradually 
pouring  water  in,  the  moment  the  level  of  the  liquid  just  touches 
the  point  of  the  rod,  o,  the  disc,  a,  begins  to  allow  some  water  to 


-83] 


Hydraulic  Press. 


escape.  The  same  result  ensues  if  for  the  straight  tube,  P,  the 
inclined  one  Q,  be  substituted.  In  these  three  cases,  therefore,  pro- 
vided the  height  of  the  liquid  is  the  same,  the  pressure  on  the  disc, 
a,  is  the  same,  whatever  be  the  shape  and  capacity  of  the  vessels. 

Moreover,  the  weight  which  has  to  be  put  on  the  scale-pan  to 
establish  equilibrium,  shows  that  the  pressure  exerted  by  the  liquid 
is  equal  to  the  weight  of  a  column  of  water,  the  base  of  which  is 
the  internal  section  of  the  ttibulure,  c,  and  the  height  the  vertical 
distance  from  the  disc  to  the  surface  of  the  liquid. 

This  principle  is  sometimes  called  the  hydrostatical  paradox, 
for  at  first  sight  it  seems  quite  impossible. 

82.  Pascal's     experiment.  —  Pascal 
made    the  following  experiment,   which 
proves  what  great  pressures  may  be  pro- 
duced by  even  small  quantities  of  liquid 
when  contained  in  vessels  of  great  height. 
He   fixed    firmly,  in    a    stout    cask,   as 
represented  in  fig.  62,  a  very  narrow  tube 
about  30  feet  in  height,  and  then  filled 
the  cask  and  the  tube  with  water.     The 
effect  of  this  was  to  burst  the  cask  :  for 
there  was  a  pressure  on   the  bottom  of 
the  cask  equal  to  the  weight  of  a  column 
of   water  whose   base   was    the   bottom 
itself,  and  whose  height  was  equal  to  that 
of  the  water  in  the  tube  (81). 

83.  Hydraulic  press. — The  law  of  the 
equality  of  pressure  has  received  a  most 
important   application    in   the   hydraulic 
press,   a    machine    by   which    enormous 
pressures  may  be  produced.    Its  principle 
is  due  to   Pascal,  but   it  was  first   con- 
structed by  Bramah  in  1796. 

Fig.  63  represents  an  elevation,  and 
fig.  64  a  section  of  the  instrument  ;  it 
consists  of  two  iron  cylinders  or  barrels, 
A  and  B,  of  unequal  diameters.  In  the 
barrel  A,  which  is  of  very  small  diameter, 
is  a  cylindrical  rod,  a,  which  acts  as 
piston,  and  can  be  moved  up  and  down 
by  the  lever,  O.  In  the  cylinder,  B,  the  internal  diameter  of  which 


Hydrostatics. 


[83- 


is  12  to  15  times  that  of  the  barrel,  A,  is  a  long  cylindrical  iron 
ram,  C,  which  also  forms  a  piston,  and  works  water-tight  in  the 
barrel  B.  On  the  top  of  the  ram,  C,  is  an  iron  slab,  K,  which 
rises  and  falls  with  it.  Four  wrought-iron  columns  support  a 
second  plate,  MN,  which  is  fixed.  The  objects  to  be  pressed  are 
placed  between  K  and  MN. 


When  the  piston  is  raised  by  means  of  the  level,  a  vacuum  is 
produced  in  the  barrel  A,  and  a  valve,  S,  at  the  bottom  opens  and 
allows  water  to  pass  from  a  reservoir,  P,  into  the  barrel.  When 
a  re-descends,  the  valve,  S,  closes  ;  but  another  valve,  ;;z,  placed 
at  the  bottom  of  the  tube  d,  opens ;  the  water  is  thus  forced  by 
this  tube  into  the  large  cylinder,  B.  At  the  next  stroke  of  the 
piston,  #,  a  fresh  quantity  of  water  is  drawn  from  the  reservoir,  P, 
and  forced  into  the  barrel  B,  and  so  forth. 


-83] 


Hydraulic  Press. 


73 


In  consequence  of  the  principle  of  the  equality  of  pressure,  the 
downward  pressure  exerted  by  the  small  piston,  a,  is  transmitted 
upwards  upon  the  piston  C.  The  pressure  which  can  be  obtained 
depends  on  the  relation  of  the  piston  C  to  that  of  the  piston  a.  If 
the  former  has  a  transverse  section  fifty  or  a  hundred  times  as  large 
as  the  latter,  the  upward  pressure  on  the  large  piston  will  be  fifty 
or  a  hundred  times  that  exerted  upon  the  small  one.  By  means  of 
the  lever,  O,  an  additional  advantage  is  obtained.  If  the  distance 
from  the  fulcrum  to  the  point  where  the  power  is  applied  is  five 
times  the  distance  from  the  fulcrum  to  the  piston,  a7  the  pressure 
on  a  will  be  five  times  the  power.  Thus,  if  a  man  acts  on  O  with 


a  force  of  sixty  pounds,  the  force  transmitted  by  the  piston  a  will 
be  300  pounds,  and  the  force  which  tends  to  raise  the  piston  C  will 
be  30,00x3  pounds,  supposing  the  section  of  C  is  a  hundred  times 
that  of  a. 

The  hydraulic  press  is  used  in  all  cases  in  which  great  pressures 
are  required.  It  is  used  in  pressing  cloth,  in  extracting  the  juice 
of  beet  root,  in  expressing  oil  from  seeds,  and  in  pressing  apples  in 
making  cider  ;  it  also  serves  to  test  the  strength  of  cannon,  of 
steam  boilers,  and  of  chain  cables.  The  parts  composing  the 
tubular  bridge  which  spans  the  Menai  Straits  were  raised  by 
means  of  an  hydraulic  press.  The  cylinder  of  this  machine,  the 
largest  which  has  ever  been  constructed,  was  nine  feet  long  and 
twenty-two  inches  in  internal  diameter ;  it  was  capable  of  raising 
a  weight  of  two  thousand  tons. 


74  Hydrostatics.  [84  - 


CHAPTER     II. 

EQUILIBRIUM   OF   LIQUIDS. 

84.  Conditions  of  the  equilibrium  of  liquids. — We  have  seen 
that  the  conditions  of  the  equilibrium  of  a  solid  are  that  its  centre 
of  gravity  be  supported  by  a  fixed  point ;  all  the  other  parts  of  the 
body  then  retain  the  same  state  of  equilibrium  in  consequence  of 
cohesion,  which  unite  the  particles  to  each  other,  and  to  the  centre 
of  gravity.  This  is  by  no  means  the  case  with  liquids ;  owing  to 
the  greater  mobility  of  their  molecules,  and  the  facility  with  which 
they  obey  the  force  of  gravity,  they  would  flow  away  and  spread 
out  in  a  horizontal  position,  if  they  were  not  retained  by  some 
obstacle.  Hence  a  liquid  cannot  be  at  rest  in  any  vessel,  unless  it 
satisfies  the  following  conditions  :— 

I.  The  free  surface  of  the  liquid  must  be  horizontal,  that  z's, 
perpendicular  everywhere  to  the  direction  of  gravity. 

II.  Every  molecule  of  the  mass  of  the  liquid  must  be  subject  in 
every  direction  to  equal  and  contrary  pressures. 

The  second  condition  is  self-evident  ;  for  if,  in  two  opposite 
directions,  the  pressures  exerted  on  any  given  molecule  were  not 
equal  and  contrary,  the  molecule  would  be  moved  in  the  direction 
of  the  greater  pressure,  and  there  would  be  no  equilibrium.  Thus 
the  second  condition  follows  from  the  principle  of  the  equality  of 

pressures,  and  from  the  reaction  which 
all  pressure  causes  on  the  mass  of 
liquids. 

To  account  for  the  first  condition 
relative  to  the  free  surface  of  the  liquid, 
let  us  observe  that  in  a  liquid  whose 
Fj    6s  surface  is  horizontal,  all  the  molecules 

supporting  each  other,  the  action   of 

gravity  is  destroyed,  and  the  liquid  is  at  rest.  But  if  the  surface 
is  not  horizontal,  if  some  parts  are  higher  than  others  (fig.  65), 
the  higher  part,  ad,  exerts  upon  any  horizontal  layer,  bdt  a  greater 
pressure  than  the  part  cd,  and  therefore  as  a  given  molecule,  o, 
of  the  horizontal  layer  is  exposed  to  a  greater  pressure  in  the 
direction  bo  than  in  the  direction  do,  equilibrium  is  impossible. 


-85] 


L  cvcl  of  L  iqitids. 


75 


In  saying  that  in  order  that  a  liquid  be  at  rest  its  surface  must 
be  horizontal,  we  must  remark  that  that  presumes  the  liquid  only 
to  be  acted  upon  by  gravity,  which  is  usually  the  case  ;  if  it  is 
under  the  action  of  other  forces,  as  is  the  case  with  the  capillary 
phenomena,  where  it  is  attracted  by  the  sides  of  the  vessel,  its 
surface  is  then  inclined  so  as  to  be  perpendicular  to  the  resultant 
of  the  forces  which  act  upon  it. 

85.  Level  of  liquids. — A  liquid  is  said  to  be  level  when  all  the 
points  of  its  surface  are  in  the  same  horizontal  plane.  This,  how- 
ever, only  applies  to  surfaces  of  small  extent.  For  as  the  direction 
of  the  vertical  constantly  changes  from  one  place  to  another  on  the 


Fig.  66. 

surface  of  the  globe,  the  direction  of  the  horizontal  surfaces  changes 
too  ;  that  is  to  say,  that  a  plane  which  is  horizontal  at  one  part  of 
the  earth's  surface,  is  not  parallel  to  a  horizontal  plane  at  a  small 
distance  ;  they  form  an  angle  with  each  other.  Hence  a  liquid 
surface  of  some  extent  in  a  state  of  equilibrium,  being  necessarily 
horizontal  in  each  of  its  parts,  does  not  form  one  single  perfectly 
plane  surface,  but  a  series  of  plane  surfaces  inclined  to  each  other ; 
which  of  course  produces  a  curved  surface.  This  curvature  cannot, 
however,  be  perceived  on  surfaces  of  small  extent,  as  in  water 
contained  in  a  vessel ;  for  the  surface  of  such  a  liquid  is  so  per- 
fectly levelled,  that  it  reflects  the  rays  of  light  like  the  most  per- 


76 


Hydrostatics. 


[85- 


fectly  polished  plane  mirror.  The  curvature  is,  however,  easily 
observed  on  large  surfaces  like  those  of  the  sea.  For  if  this  surface 
were  perfectly  level,  a  ship  in  sailing  away  from  the  shore  would 
only  cease  to  be  visible  in  consequence  of  increasing  distance,  and 
the  less  apparent  parts,  the  masts  and  the  cordage,  would  disappear 
first.  This,  however,  is  not  the  case  ;  the  hull  first  sinks  below  the 
horizon,  then  the  lower  part  of  the  masts,  and  ultimately  the  top,  as 
seen  in  fig.  66,  thus  proving  the  curvature  of  the  surface  of  the  sea. 

86.  True  and  apparent  level. — When  we   consider   a   great 
surface  of  water— the  Mediterranean  sea,  for  instance — its  surface 
is  said  to  be  level  when  all  points  of  the  surface  are  equidistant 
from  the  centre  of  the  earth.      This  is  the  true  level ;  while  that 
level  which  is  defined  as  having  all  the  points  of  its  surface  in  the 
same  horizontal  plane,  is  the  apparent  level,  the  level  for  the  eye. 
The  true  level  only  coincides  with  the  apparent  level  when  the  liquid 
surfaces  are  very  small.     If  the  earth  did  not  rotate  about  its  own 
axis,  the  surface  of  all  seas  would  form  a  true  level ;  but  owing  to  the 
centrifugal  force  which  results  from  its  daily  motion,  the  surface  is 
heaped  up  at  the  equator,  and  the  level  is  higher  than  at  the  poles. 

87.  Equilibrium  of  the  same  liquid  in    several    communi- 
cating vessels. — Not  merely  do  liquids   tend   to   become  level 


Fig.  67. 

when  they  are  placed  in  the  same  vessel,  but  also  when  they  are 
placed  in  vessels  which  communicate  with  each  other.      Whatever 


-88] 


Equilibrium  of  L  iquids. 


77 


the  shape  and  the  dimensions  of  these  vessels,  equilibrium  will 
exist,  when  the  surfaces  of  the  liqidds  in  all  the  vessels  are  in  the 
same  horizontal  plane. 

This  principle  may  be  demonstrated  by  means  of  the  apparatus 
represented  in  fig.  67.  It  consists  of  a  series  of  vessels  of  different 
shapes  and  capacities  connected  together  by  a  common  horizontal 
tubulure.  When  water  or  any  other  liquid  is  poured  into  the 
vessel,  the  level  is  seen  to  rise  at  the  same  time,  and  stop  at 
exactly  the  same  height  in  each.  Equilibrium  is  then  established. 
For  as  we  have  seen  that  the  pressures  exerted  by  a  liquid  do  not 
depend  upon  its  quantity  but  upon  its  height  (81),  when  this  is 
the  same  for  all  the  vessels  above  the  tube  of  communication  abc, 
the  pressure  is  necessarily  everywhere  equal,  and  therefore,  as  the 
liquid  has  no  more  tendency  to  flow  from  b  towards  a  than  from 
b  to  c,  equilibrium  continues. 

88.  Equilibrium  of  different  liquids  in  communicating- 
vessels. — In  what  has  been  said  the  communicating  vessels  all 


Fig.  68. 

contained   the   same  liquid.     It  may,  however,  happen   that  the 
vessels   contain  liquids  of  different  densities,  which  do  not  mix. 


Hydrostatics. 


[88- 


The  level  is  then  no  longer  the  same  ;  the  lighter  liquids  are 
higher,  and  equilibrium  is  only  possible  when  the  heights  of  the 
liquid  columns  in  communication  are  inversely  as  their  densities ; 
that  is,  that  if  one  of  the  liquids  is  twice  or  thrice  as  dense  as 
another,  its  height  will  be  half  or  one-third  as  much. 

This  principle  is  demonstrated  experimentally  by  means  of  the 
apparatus  represented  in  fig.  68.  It  consists  of  two  glass  tubes 
connected  at  the  bottom  by  a  narrow  tube.  The  tubes  are  sup- 
ported by  two  vertical  columns,  and  on  each  of  them  is  a  scale 
graduated  on  the  glass  itself.  If  then  mercury  is  poured  into  one 
of  the  tubes,  it  quickly  assumes  the  same  level  in  each.  On  now 
pouring  water  into  the  tube  A,  the  level  of  the  mercury  is  seen  to 
sink  in  this  tube  in  virtue  of  the  pressure  of  the  water,  and  it  rises 
in  the  other  tube.  Then,  when  equilibrium' is  established  the' 
mercury  in  B  is  higher  than  in  the  tube  A  by  a  quantity,  cd.  It 
is  clear,  then,  that  the  pressure  of  the  column  of  mercury,  cd,  counter- 
balances the  pressure  of  the  column  of  water,  ab.  If  now  the 
heights  of  ab  and  cd  be  measured  by  means  of  the  graduated  scales 
on  the  two  tubes,  it  will  be  found  that  the  height  cd  is  13-6  as 
small  as  that  of  ab  ;  which  demonstrates  the  above  principle,  for 
we  shall  presently  see  that  mercury  is  13-6  times  as  heavy  as 
water. 

89.  Equilibrium  of  superposed 
liquids. — In  order  that  there  should  be 
equilibrium  when  several  heterogeneous 
liquids  which  do  not  mix  are  superposed 
in  the  same  vessel,  each  of  them  must 
satisfy  the  conditions  necessary  for  a  single 
liquid  ;  and  further,  there  will  be  a  stable 
equilibrium  only  when  the  liquids  are 
arranged  in  the  order  of  their  decreasing 
densities  from  the  bottom  upwards. 

The  last  condition  is  experimentally 
demonstrated  by  means  of  the  phial  of 
four  elements  (fig.  69).  It  consists  of  a 
long  narrow  bottle  containing  mercury, 
water  saturated  with  carbonate  of  potass, 
alcohol  coloured  red,  and  petroleum. 
When- the  phial  is  shaken  the  liquids  mix, 
but  when  it  is  allowed  to  rest  they 
separate  ;  the  mercury  sinks  to  the  bottom,  then  comes  the  water, 


Fig.  69. 


-90]  Water  Level.  79 

then  the  alcohol,  and  then  the  petroleum.  This  is  the  order  of  the 
decreasing  densities  of  the  bodies.  The  water  is  saturated  with 
carbonate  of  potass  to  prevent  its  mixing  with  the  alcohol. 

This  separation  of  the  liquids  is  due  to  the  same  causes  as  that 
which  enables  solid  bodies  to  float  on  the  surface  of  a  liquid  of 
greater  density  than  their  own.  It  is  also  from  this  principle  that 
fresh  water,  at  the  mouths  of  rivers,  floats  for  a  long  time  on  the 
denser  salt  water  of  the  sea  ;  and  for  the  same  reason  cream,  which 
is  lighter  than  milk,  rises  to  the  surface. 


APPLICATIONS   OF  THE   PRINCIPLE   OF  THE   EQUILIBRIUM 
OF  LIQUIDS. 

90.  "Water  level. —  In  a  great  number  of  operations,  such  as  the 
construction  of  canals,  railways,  roads,  etc.,  it  is  continually  neces- 
sary to  determine  the  difference  in  level  of  two  more  or  less  distant 
places.  The  simplest  apparatus  for  this  purpose  is  the  water  level 


Fig.  70. 

which  is  an  application  of  the  conditions  of  equilibrium  in  commu- 
nicating vessels.  It  consists  of  a  metal  tube  bent  at  both  ends,  in 
which  are  fitted  glass  tubes  (fig.  70).  It  is  placed  on  a  tripod,  and 
water  poured  in  the  tube  until  it  rises  in  both  limbs.  When  the 
liquid  is  at  rest,  the  level  of  the  water  in  both  tubes  is  the  same — 
that  is,  they  are  both  in  the  same  horizontal  plane. 

This  instrument  is  used  in  levelling,  or  ascertaining  how  much 
one  point  is  higher  than  another.      If,  for  example,  it  is  desired  to 


•m  m 


So  Hydrostatics.  [90- 

find  the  difference  between  the  heights  of  two  places,  a  levelling- 
stafl"\s  fixed  on  the  latter  place.  This  staff  consists  of  a  rule  formed 
of  two  sliding  pieces  of  wood,  one  of  which  supports  a  piece  of  tin 
plate,  in  the  centre  of  which  there  is  a  mark.  This  staff  being  held 
vertically,  an  observer  looks  at  it  through  the  level  along  the  surfaces 
in  the  two  tubes,  and  directs  the  holder  to  raise  or  lower  the  slide 
until  the  mark  is  in  the  prolongation  of  the  level  in  the  two  tubes. 
The  assistant  then  reads  off  on  the  graduated  rod  the  height  of 
the  mark  upon  the  ground.  If  this  height  exceeds  that  of  the  level, 
the  height  of  the  latter  is  subtracted  from  that  of  the  former,  and  the 
difference  gives  the  difference  in  the  heights  of  the  two  places. 

91.  Spirit  level. — The  spirit  level  is  both  more  delicate  and 
more  accurate  than  the  water  level.  It  consists  of  a  glass  tujpe 
(fig.  71),  very  slightly  curved  ;  it  is  filled  with  spirit  with  the  ex- 


Fig.  7i. 

ception  of  a  bubble  of  air,  which  tends  to  occupy  the  highest  part. 
The  tube  is  placed  in  a  brass  case,  which  is  so  arranged  that  when 
it  is  in  a  perfectly  horizontal  position  the  bubble  of  air  is  exactly 
between  the  two  points  marked  in  the  case.  But  if  the  plane  on 
which  the  instrument  rests  is  ever  so  little  inclined,  the  air  bubble 
tends  to  move  towards  the  higher  part. 

This  thus  furnishes  a  ready  means  of  ascertaining  whether  any 
article — a  table,  a  stand,  or  a  bookshelf — is  quite  horizontal. 

To  take  levels  with  this  apparatus,  it  is  fixed  on  a  telescope, 
which  can  consequently  be  placed  in  a  horizontal  position. 

92.  Jets  of  water. — The  jets  which  ornament  our  gardens  and 
public  places,  depend  on  the  tendency  of  liquids  always  to  become 
level.  For  the  water  which  jets  out  always  comes  from  a  reservoir 
placed  in  a  higher  position  than  that  where  the  jet  is;  and  its  jetting 
is  a  consequence  of  its  tendency  to  form  a  level.  Fig.  72  gives  an 
idea  of  this  phenomenon.  On  the  eminence  on  the  left  of  the  figure 
is  a  reservoir  containing  water,  from  the  bottom  of  which  passes  a 


-93] 


Streams,  Springs,  Wells. 


81 


tube  which  terminates  in  the  centre  of  the  basin.     The  water  then 
jets  out,  forced  by  the  pressure  of  a  column  of  water,  the  height  of 


Fig.  72. 

which  is  equal  to  the  difference  in  level  between  the  reservoir  and 
the  basin. 

Theory  proves  that  in  such  a  case  the  water  always  tends  to 
rise  to  the  level  of  the  reservoir  from  which  it  is  supplied.  It  never 
attains  this  freight,  for  the  jet  experiences  three  kinds  of  resistances  : 
ist,  the  friction  of  the  water  in  the  conduit  pipe  ;  2nd,  the  resist- 
ance of  the  air ;  and  3rd  the  hindrance  offered  by  the  particles 
falling  from  the  height  of  the  jet  upon  those  ascending. 

93.  Streams,  springs,  wells. — The  formation  of  springs  upon 
the  surface  of  the  earth,  and  in  its  interior,  is  also  due  to  the  tendency 
of  water  to  seek  its  level.  For  gravity  causes  water  to  flow  from 
higher  to  lower  places.  Hence  it  is  that  the  rain  which  falls  upon 
the  earth,  and  the  water  arising  from  the  melting  of  snow,  pass 
down  to  the  valleys,  where  they  form  brooks,  streams,  and  rivers, 
which  flow  along  their  beds  as  along  an  inclined  plane,  until  they 
emerge  into  the  seas.  A  very  small  fall  can  give  rise  to  a  current. 
Thus  the  mean  height  of  the  Seine  at  Paris  is  not  more  than  35 
yards  above  the  sea-level.  The  extent  of  its  course  between  these 
two  points  is  about  224  miles,  which  scarcely  amounts  to  a  fall  of 

G 


82 


Hydrostatics. 


[93- 


the  3 f  gth  part  of  an  inch  in  a  yard  ;  and  water  requires  several  days 
to  traverse  this  distance. 

All  the  rain  which  falls  does  not  flow  upon  the  surface  ;  part  of 
it  penetrates  into  the  earth,  and  gives  rise  to  small  subterranean 
watercourses  which  are  called  springs.  It  is  in  order  to  procure 
water  from  these  that  wells  are  sunk. 

94.  Artesian  wells. — When  the  spring  which  feeds  a  well  comes 
from  a  place  much  higher  than  that  where  the  well  is  sunk,  it  may 
happen  that  water  tends  to  rise  higher  than  the  ground.  This  is 
what  happens  in  what  are  called  Artesian  wells.  These  wells  derive 
their  name  from  the  province  of  Artois,  where  it  has  long  been  cus- 
tomary to  dig  them,  and  from  whence  their  use  in  other  parts  of 


Fig.  73- 

France  and  Europe  was  derived.  It  seems  however,  that  at  a  very 
remote  period,  wells  of  the  same  kind  were  dug  in  China  and  Egypt. 
To  understand  the  theory  of  these  wells,  it  must  be  premised 
that  the  strata  composing  the  earth's  crust  are  of  two  kinds  :  the 
one  permeable  to  water,  such  as  sand,  gravel,  etc. ;  the  other 
impermeable,  such  as  clay.  Let  us  suppose,  then,  a  basin  of 
greater  or  less  extent,  in  which  the  two  impermeable  layers  AB, 
CD  (fig.  73),  enclose  between  them  a  permeable  layer  KK.  The 
rain-water  falling  on  the  part  of  this  layer  which  comes  to  the 
surface,  which  is  called  the  outcrop, -will  filter  through  it,  and, 
following  the  natural  fall  of  the  ground,  will  collect  in  the  hollow  of 
the  basin,  whence  it  cannot  escape,  owing  to  the  impermeable  strata 
above  and  below  it.  If  now  a  vertical  hole,  I,  be  sunk  down  to  the 
water-bearing  stratum,  the  water  striving  to  regain  its  level  will  spout 


-95] 


Bodies  immersed  in  Liquids. 


out  to  a  height  which  depends  on  the  difference  between  the  levels 
of  the  outcrop  and  of  the  point  at  which  the  perforation  is  made. 

The  waters  which  feed  Artesian  wells  often  come  from  a  distance 
of  sixty  or  seventy  miles,  The  depth  varies  in  different  places. 
The  well  at  Crenelle  is  1,800  feet  deep  ;  it  gives  656  gallons  of 
water  in  a  minute,  and  is  one  of  the  deepest  and  most  abundant 
which  has  been  made.  The  temperature  of  the  water  is  27°  C.  It 
follows  from  the  law  of  the  increase  of  temperature  with  the  in- 
creasing depth  below  the  surface  of  the  ground  (297),  that,  if  this 
well  were  210  feet  deeper,  the  water  would  have  all  the  year  round 
a  temperature  of  32°  Cv  that  is,  the  ordinary  temperature  of  warm 
baths. 


CHAPTER  III. 

PRESSURES  SUPPORTED   BY  BODIES   IMMERSED   IN  LIQUIDS. 
SPECIFIC  GRAVITIES.     AREOMETERS. 

95.  Pressure  supported  by  a  body  immersed  in  a  liquid. — 

When  a  solid  is  immersed  in  a  liquid,  it  is  obvious  that  the  pressures 
which  the  sides  of  the  vessel  support  are  also  exerted  against  the 
surface  of  the  body  immersed,  since  liquids  transmit  pressure  in  all 
directions  (76),  But  it  is  readily  seen  that  the  pressures  which  the 
immersed  body  supports  do  not  neutral- 
ise themselves,  but  have  a  resultant,  the 
tendency  of  which  is  to  move  the  body 
upwards. 

Let  us  imagine  a  cube  immersed  in  „ 
a  mass  of  water  (fig,  74),  and  that  four  of 
its  edges  are  vertical.     The  horizontal 
pressures  upon  the  two  opposite  faces, 
a  and  <£,  are  clearly  of  the  same  inten- 
sity, for  they  are  exerted  at  the  same 
depth  (78) ;  and  as  they  are  in  opposite    ( 
directions  they  will  balance  one  an- 
other, and  the  only  effect  will  be  to  com- 
press the  body  without  displacing  it. 

But  the  vertical  pressures  on  the  faces  *fand  c  are  obviously  un- 
equal.    The  first  is  pressed  downwards  by  a  column  of  water  whose 

o  2 


Fig.  74. 


Hydrostatics. 


[95- 


base  is  the  face  d,  and  whose  height  is  dn,  the  lower  face  c  is  pressed 
upwards  by  the  weight  of  a  column  of  water  whose  base  is  the  face 
itself,  and  whose  height  is  en.  The  cube,  therefore,  is  urged  up- 
wards by  a  force  equal  to  the  difference  between  these  two  pressures, 
which  latter  is  manifestly  equal  to  the  weight  of  a  column  of  water 
having  the  same  base  and  the  same  height  as  this  cube.  By  this 
reasoning,  therefore,  we  arrive  at  the  remarkable  principle,  that  any 
body  immersed  in  a  liquid  is  pressed  upwards  by  a  pressure  equal 
to  the  weight  of  the  volume  of  liquid  which  it  displaces.  We  shall 
see  how  this  principle  can  be  experimentally  verified. 

96.  Principle  of  Archimedes.  Hydrostatic  balance. — We 
have  thus  seen  that  any  body  immersed  in  a  liquid  is  submitted  to 
the  action  of  two  forces — gravity  which  tends  to  make  it  sink,  and 


.    Fig,  75- 

the  buoyancy  of  the  liquid  which  tends  to  raise  it  with  a  force  equal 
to  the  weight  of  the  liquid  displaced.  The  body  weighs  less  there- 
fore than  in  air,  and  the  diminution  of  its  weight  is  exactly  equal  to 
the  weight  of  the  displaced  liquid.  The  above  principle  may  be 


-96]  Hydrostatic  Balance.  85 

thus  enunciated  :  that  a  body  immersed  in  a  liquid  loses  a  part  of 
its  weight  equal  to  the  weight  of  the  displaced  liquid.  For  instance, 
suppose  that  a  body  which  in  air  weighs  1,000  grains,  when  im- 
mersed in  water  displaces  a  cubic  inch  of  water  ;  it  will  now  only 
weigh  1,000-252  =  748  grains  (a  cubic  inch  of  water  =  252  grains). 

This  principle,  which  is  remarkable  for  its  numerous  applications, 
is  called  the  l  principle  of  Archimedes,'  after  the  discoverer.  It  is 
shown  experimentally  by  means  of  the  hydrostatic  balance  (fig.  75). 
This  is  an  ordinary  balance,  each  pan  of  which  is  provided  with  a 
hook ;  the  rod,  c,  slides  in  the  hollow  cylinder  d.  The  beam  is  sup- 
ported on  the  rod,  c,  which  can  be  fixed  in  any  position  by  means  of 
a  screw,  n.  The  beam  being  raised,  a  hollow  brass  cylinder,  £,  is 
suspended  to  one  of  the  pans,  and  below  this  a  solid  cylinder,  a, 
whose  volume  is  exactly  equal  to  the  capacity  of  the  first  cylinder  ; 
lastly  an  equipoise  is  placed  in  the  other  pan.  If  now  the  hollow 
cylinder,  a,  be  filled  with  water,  the  equilibrium  is  disturbed,  but  if  at 
the  same  time  the  beam  is  lowered  so  that  the  solid  cylinder 
becomes  immersed  in  a  vessel  of  water  placed  beneath  it,  the  equi- 
librium will  be  restored.  By  being  immersed  in  water,  the  cylinder 
a  loses  a  part  of  its  weight  equal  to  that  of  the  water  in  the  cylinder 
b.  Now  as  the  capacity  of  the  cylinder  a  is  exactly  the  same  as 
that  of  the  cylinder  £,  the  principle  which  has  been  laid  down  is 
proved. 

It  is  stated  that  Archimedes  discovered  this  principle  on  the 
occasion  of  a  problem  which  had  been  propounded  to  him  by  Hiero, 
tyrant  of  Syracuse.  This  prince,  desiring  to  offer  to  Jupiter  a  gold 
crown,  had  furnished  a  goldsmith  with  ten  pounds  of  gold  as  the 
material  for  this  purpose.  The  crown  when  finished  was  found  to 
weigh  ten  pounds,  but  Hiero,  suspecting  that  some  of  the  gold  had 
been  replaced  by  silver,  owing  to  the  beauty  of  its  workmanship, 
demanded  from  Archimedes  a  means  of  detecting  the  supposed 
fraud  without  destroying  the  crown. 

Archimedes  pondering  over  the  solution  of  the  problem,  was  in 
the  bath,  when  he  observed  that  he  could  raise  his  limbs  in  water 
more  easily  than  in  air.  This  simple  observation  was  a  gleam  of 
light  for  him  ;  he  discovered  the  above  principle,  and  this  led  him 
to  a  simple  means  of  calculating  the  quantity  of  gold  and  silver  in 
the  crown.  It  is  said  that  Archimedes  was  so  transported  with  joy, 
at  his  discovery  that  he  ran  home  from  the  bath,  crying  in  the 
streets,  EvprjKn,  iviifKa  (I  have  found  it). 

We  have  all  had  occasion  to  make  the  observation  of  Archimedes, 


86  Hydrostatics.  [96- 

on  observing  how  much  lighter  our  limbs  appear  in  water,  and  on 
the  contrary,  how  much  heavier  they  seem  when  lifted  out.  In  like 
manner,  if  the  body  is  almost  entirely  immersed  in  water,  we  can 
walk  barefoot  on  the  stones  without  injuring  the  feet  ;  but  this  is 
not  possible  when  we  are  out  of  the  water.  For  in  the  former  case 
part  of  the  weight  of  the  body  is  raised  by  the  liquid,  while  in  the 
latter  the  whole  weight  of  the  body  presses  the  feet  against  the 
sharp  projections. 

97.  Equilibrium  of  immersed  and  floating:  bodies. — When  a 
body  is  placed  in  a  liquid,  three  cases  are  possible  :  the  body  may 
have  the  same  specific  gravity  as  the  liquid,  in  which  case  it  weighs 
as  much  as  the  liquid  for  an  equal  volume  ;  or  it  may  be  denser,  in 
which  case  it  weighs  more  ;  or  it  is  lighter,  and  in  this  case  it 
weighs  less. 

I.  If  the  body  immersed  is  of  the  same  density  as  the  liquid,  the 
weight  of  the  liquid  displaced  being  the  same  as  that  of  the  body, 
it  follows  from  Archimedes'  principle  that   the  buoyancy  which 
tends  to  raise  it,  is  exactly  equal  to  the  force  with  which  gravity 
tends  to  sink  it.     The  two  forces  are  thus  in  equilibrium,  and  the 
body  remains  in  suspension  in  any  position  in  the  liquid. 

II.  If  the  body  immersed  is  denser  than  the  liquid,  it  sinks,  for 
then  its  weight  preponderates  over  the  buoyancy.     This  is  the  case 
when  a  stone  or  a  mass  of  metal  is  thrown  into  water. 

III.  Lastly,  if  the  immersed  body  is  lighter  than  the  liquid,  the 
buoyancy  prevails,  and  the  body  rises  until  it  only  displaces  a 
weight  of  liquid  equal  to  its  own.     It  is  then  said  to  float.     Cork, 
wax,  wood,  and  all   substances  lighter  than  water,  float  on   its 
surface. 

A  body  which  floats  on  one  liquid  may  sink  in  another;  the  body 
for  this  purpose  must  be  lighter  than  the  one  liquid,  but  heavier 
than  the  other.  An  egg  sinks  at  once  if  placed  in  ordinary  water, 
since  it  is  heavier  than  an  equal  volume  of  water  ;  but'  it  swims  if 
placed  in  strong  brine,  which  is  denser  than  water.  A  piece  of  oak 
floats  on  water,  but  sinks  in  ether,  which  is  lighter  than  water.  Iron 
floats  on  mercury,  but  sinks  at  once  in  water. 

Yet  a  body,  though  denser  than  a  liquid  may  float  on  its  surface. 
For  this  purpose  it  must  have  such  a  shape  as  to  displace  a  volume 
of  liquid,  the  weight  of  which  is  greater  than  its  own.  Porcelain  is 
much  heavier  than  water,  yet  a  porcelain  saucer  placed  on  water 
floats  on  the  surface  ;  this  arises  from  its  concave  shape,  owing 
to  which  it  displaces  a  weight  of  water  equal  to  its  own,  though  it  is 


-100] 


Swimming. 


only  partially  immersed.    For  the  same  reason  iron  ships,  even  with 
very  thick  sides,  float  freely  on  water. 

98.  Cartesian  diver. — The  different  effects  of  suspension,  im- 
mersion, and  floating  are  reproduced  by  means  of  a  well-known 
hydrostatic  toy,  the  Cartesian  diver  (fig.  76).     It  consists  of  a  glass 
cylinder,  nearly  full  of  water,  on  the  top  of  which  a  brass  cap,  A, 
provided  with  a  piston,  is  hermetically  fitted.     In  the  liquid  there  is 
a  little  porcelain  figure,  a  fish,  0,  for  example,  attached  to  a  hollow- 
glass  ball,  m,  which  contains  air  and  water,  and  floats  on  the  sur- 
face.    In  the  lower  part  of  this  figure  there  is  a  little  hole  by  which 
water  can  enter  or  escape,  according  as  the  air  in  the  interior  is 
more  or  less  compressed.     The  quantity  of  water  in  the  globe  is 
such,  that  very  little  more  is  required  to  make  it  sink.     If  the  piston 
be  slightly  lowered  the  air  is   compressed,  and  this  pressure  is 
transmitted  to  the  water  of  the  vessel 

and  to  the  air  in  the  bulb.  The  con- 
sequence is,  that  a  small  quantity  of 
water  penetrates  into  the  bulb,  which 
therefore  becomes  heavier  and  sinks. 
If  the  pressure  is  relieved,  the  air  in  the 
bulb  expands,  expels  the  excess  of 
water  which  had  entered  it,  and  the 
apparatus  being  now  lighter,  rises  to 
the  surface.  The  experiment  may  also 
be  made,  by  replacing  the  brass  cap 
and  piston  by  a  cover  of  sheet  india 
rubber,  which  is  tightly  tied  over  the 
mouth.  When  this  is  pressed  by  the 
hand  the  same  effects  are  produced. 

99.  Swimming  bladder  of  fishes. 
— Most    fishes    have    an    air-bladder 
below  the  spine,  which  is  called  the 
swimming    bladder.       The    fish    can 
compress  or  dilate  this  at  pleasure  by 
means  of  a  muscular  effort,  and  pro- 
duce the  same  effects  as  those  just 
described — that  is,  it  can  either  rise  or 
sink  in  water. 

100.  Swimming. — The  human  body 

is  lighter,  on  the  whole,  than  an  equal  volume  of  water  ;  it  conse- 
quently floats  on  the  surface  and  still  better  in  sea  water,  which  is 


88 


Hydrostatics. 


[100- 


heavier  than  fresh  water.  The  difficulty  in  swimming  consists,  not 
so  much  in  floating,  as  in  keeping  the  head  above  water,  so  as  to 
breathe  freely.  In  man  the  head  is  heavier  than  the  lower  parts, 
and  consequently  tends  to  sink,  and  hence  swimming  is  not 
natural  to  him,  but  is  an  art  which  requires  to  be  learned.  With 
quadrupeds  on  the  contrary,  the  head  being  less  heavy  than  the 
posterior  part  of  the  body,  remains  above  water  without  any  effort, 
and  these  animals  therefore  swim  naturally. 

If  a  person  who  cannot  swim,  and  who  falls  into  the  water,  retains 


Fig.  77. 

coolness  enough  to  turn  on  his  back,  so  that  his  face  is  out  of  water, 
he  can  breathe  freely,  and  wait  until  help  arrives.  Instead  of  this, 
however,  he  generally  attempts  to  raise  his  arms  out  of  water,  as  if 
grasping  at  some  fixed  support.  This  is  very  dangerous,  for  as  the 
arms  no  longer  displace  a  quantity  of  liquid  equal  to  their  own  bulk, 
their  weight  is  not  diminished  to  that  extent,  but  concurs  with  that 
of  the  head  in  making  them  sink. 

Weight  for  weight,  fat  persons  swim  more  easily  than  lean  ones, 
for  they  displace  more  water.  For  the  same  reason  air  bladders,  or 
cork  girdles,  are  fastened  to  persons  who  are  learning  to  swim 
(fig.  77),  for  then,  without  any  considerable  increase  of  weight,  they 
displace  more  water,  which  increases  the  buoyancy  and  keeps 
them  up. 

Several  kinds  of  birds,  such  as  ducks,  geese,  and  swans,  swim 
easily  on  water.  They  owe  this  property  to  a  thick  coating  of  a 
light  impervious  down  which  covers  the  lower  part  of  the  body,  so 
that  they  displace,  even  with  a  small  immersion,  a  weight  equal  to 
their  own. 


-101] 


Specific  Gravity. 


SPECIFIC  GRAVITY.      HYDROMETERS. 

101.  Specific  gravity. — Daily  experience  shows  us  that  different 
substances  have  very  unequal  weights  for  one  and  the  same 
volume.  For  instance,  we  all  know  that  gold  weighs  more  than 
silver,  lead  than  iron,  stone  than  wood.  In  order  to  compare 
equal  volumes  of  various  substances  as  to  their  weights,  the  weight 
of  water  has  been  taken  as  a  standard  of  comparison — as  unity.  For 
water  is  everywhere  met  with,  and  can  always  be  had  pure ;  this 
latter  condition  is  necessary,  for  the  weight  of  a  given  quantity  of 
water  differs  with  the  substances  it  holds  in  solution.  As,  more- 
over, the  weight  varies  with  the  temperature,  a  constant  temperature 
must  be  adopted.  Hence  the  unit  of  weight  is  distilled  water  at  a 
temperature  of  4  degrees,  for  at  this  point,  as  we  shall  afterwards 
see  (228),  water  has  its  greatest  density. 


Fig.  78- 

Thus  having  agreed  to  represent  by  I  the  weight  of  a  certain 
volume  of  distilled  water  at  4  degrees,  the  specific  gravity  of  a  body 


90  Hydrostatics.  [101- 

is  the  weight  of  the  same  'volume  of  it  as  compared  with  that  of 
water,  or  what  is  the  same,  the  number  which  expresses  how  much 
it  weighs  as  compared  with  water.  Thus,  when  we  say  that  the 
specific  gravity  of  gold  is  19,  and  that  of  lead  n,  we  mean  that  the 
former  metal  is  19  times,  and  the  latter  n  times  as  heavy  as  water. 

102.  Determination  of  the  specific  gravity  of  solids. —  Three 
methods  are  commonly  used  in  determining  the  specific  gravities  of 
solids  and  liquids.  These  are — the  method  of  the  hydrostatic 
balance,  that  of  the  hydrometer,  and  that  of  the  specific  gravity 
flask.  All  three,  however,  depend  on  the  same  principle,  that  of 
first  ascertaining  the  weight  of  a  body,  and  then  that  of  an  equal 
volume  of  water.  We  shall  first  apply  these  methods  to  determin- 
ing the  specific  gravity  of  solids,  and  then  to  the  specific  gravity  of 
liquids. 

i.  Hydrostatic  balance.  To  obtain  the  specific  gravity  of  a  solid, 
a  piece  of  iron  for  instance,  by  the  hydrostatic  balance  (fig.  78),  it  is 
first  weighed  in  air  by  suspending  it  to  the  hook  of  one  of  the 
plates.  Let  us  suppose  that  its  weight  is  585  grains.  It  is  then 
weighed  while  immersed  in  distilled  water,  as  shown  in  fig.  78.  It 
will  now  weigh  less  ;  suppose  the  weight  to  be  510  grains,  this  is  in 
accordance  with  Archimedes'  principle,  for  it  now  loses  a  weight 
equal  to  that  of  the  water  which  it  displaces.  Hence,  subtracting 
5 10  from  585.  the  difference  75  represents  the  weight  of  the  displaced 
water,  that  is,  the  weight  of  a  volume  of  water  equal  to  that  of  the 
iron  :  we  need  now  only  calculate  how  often  the  weight  75,  that  of 
the  water,  is  contained  in  585,  that  of  the  iron,  and  the  quotient  7-8 
is  the  specific  gravity  of  iron  ;  it  says  that,  for  equal  volumes,  this 
substance  weighs  7-8  times  as  much  as  water. 

Nicholson's  hydrometer.  This  apparatus  consists  of  a  hollow 
metallic  cylinder  (fig.  79),  to  which  is  fixed  a  cone,  d,  loaded  with 
lead.  The  object  of  the  latter  is  to  depress  the  centre  of  gravity 
so  that  the  cylinder  does  not  upset  when  in  the  water.  At  the  top 
is  a  stem,  c,  terminated  by  a  pan,  a,  in  which  is  placed  the  substance 
whose  specific  gravity  is  to  be  determined.  On  the  stem  a  standard 
point,  c,  is  marked. 

The  apparatus  stands  partly  out  of  the  water,  and  the  first  step 
is  to  ascertain  the  weight  which  must  be  placed  in  the  pan  in  order 
to  make  the  hydrometer  sink  to  the  standard  point  c  (fig.  80).  Let 
this  weight  be  125  grains,  and  let  sulphur  be  the  substance  whose 
specific  gravity  is  to  be  determined.  The  weights  are  then  removed 
from  the  pan,  and  replaced  by  a  piece  of  sulphur  which  weighs  less 


-102] 


Specific  Gravity  of  Solids. 


than  125  grains,  and  weights  added  until  the  hydrometer  is  again 
depressed  to  the  standard,  c.     If,  for  instance,  it  has  been  necessary 


Fig.  79. 


Fig.  80. 


to  add  55  grains,  the  weight  of  the  sulphur  is  evidently  the  difference 
between  125  and  55  grains,  that  is,  70  grains. 

Having  thus  determined  the  weight  of  the  sulphur  in  air,  it  is 
now  only  necessary  to  ascertain  the  weight  of  an  equal  volume  in 
water.  To  do  this,  the  piece  of  sulphur  is  placed  in  the  lower  pan 
at  d,  as  represented  in  fig.  81.  The  whole  weight  is  not  changed, 
nevertheless  the  hydrometer  no  longer  sinks  to  the  standard  ;  the 
sulphur,  by  immersion,  has  lost  part  of  its  weight  equal  to  that  of 
the  water  displaced.  Weights  are  added  to  the  upper  pan  until  the 
hydrometer  sinks  again  to  the  standard.  This  weight,  34-4  grains 
for  example,  represents  the  weight  of  the  volume  of  water  displaced  ; 
that  is,  of  the  volume  of  water  equal  to  the  volume  of  the  sulphur. 

It  is  only  necessary,  therefore,  to  divide  70  grains,  the  weight  in 
air,  by  34-4  grains,  and  the  quotient  2-03  is  the  specific  gravity. 

Specific  gravity  flask.  In  this  method,  which  is  advantageously 
used  for  the  determination  of  the  specific  gravity  of  bodies  in  a 
state  of  powder,  a  wide-necked  flask  is  used  which  can  be  care- 
fully closed  by  a  ground  glass  disc  (fig.  82).  Having  filled  it  with 
water  it  is  closed  with  the  disc,  great  care  being  taken  that  not 
a  bubble  of  air  is  left.  After  being  carefully  wiped  dry,  it  is  placed 
in  the  pan  of  a  balance,  and  by  its  side  is  the  substance,  a,  whose 


Hydrostatics. 


[102- 


Fig.  82. 


specific  gravity  is  to  be  determined.  The  whole  is  then  equi- 
poised by  placing  weights  in  the  other  pan  of  the  balance.  The 
substance,  a,  is  then  removed,  and  weights 
added  in  its  place,  until  equilibrium  is  again 
established.  The  weight  necessary  for  this 
purpose  gives  the  weight  of  the  substance  in 
air. 

To  obtain  its  weight  in  water  it  is  placed 
in  the  flask,  the  disc  adjusted,  and  the  whole 
again  carefully  wiped.  In  order  now  to 
equipoise  the  tare  in  the  second  pan,  weights 
must  be  added  on  the  side  of  the  flask  to 
make  up  for  the  water  displaced.  The  weights 
necessary  for  this  purpose  represent  then  the 
weight  of  a  volume  of  water  equal  to  that  of  the 
body. 

Dividing,  then,  the  weight  of  the  body  in  air  by  the  weight  of  an 
equal  volume  of  water,  we  have  the  specific  gravity  sought. 

103.  Specific  gravity  of  liquids.— These  are  determined  by  the 
same  methods  as  those  of  solids. 

Hydrostatic  balance.  In  determining  the  specific  gravity  of  a 
liquid  by  this  means,  a  body  is  suspended  to  one  of  the  pans  of  the 
balance,  which  is  neither  dissolved  by  the  liquid  whose  specific 
gravity  is  to  be  determined,  nor  by  water  ;  for  instance,  a  ball  of 
platinum,  which  is  insoluble  in  all  ordinary  liquids.  This  ball  is  first 
weighed  in  air,  then  in  water,  and  finally  in  the  liquid  in  question,, 
which  we  will  suppose  is  alcohol.  Let  us  assume  that  in  air  the  ball 
weighs  5 10  grains,  in  water  486  grains,  and  in  alcohol  489  grains.  The 
loss  of  weight  in  water  has  thus  been  510  less  486,  or  24  grains,  and 
in  alcohol  510  less  499,  or  21  grains  ;  which  tells  us  that  if  a  volume 
of  water  equal  to  that  of  the  ball  weighs  24  grains,  the  same  volume 
of  alcohol  weighs  21  grains.  Hence,  to  obtain  the  specific  weight 
of  alcohol  we  must  ascertain  how  many  times  the  number  21  con- 
tains 24,  which  of  course  is  obtained  by  division.  The  quotient 
thus  obtained  is  cr866,  which  represents  the  specific  gravity  of  alco- 
hol as  compared  with  water.  ,, 
ii.  Fahrenheit's  hydrometer.  This  instrument  (fig.  83)  resembles 
Nicholson's  hydrometer,  but  is  made  of  glass,  so  as  to  be  used  in 
all  liquids.  At  its  lower  extremity,  instead  of  a  pan,  it  is  loaded 
with  a  small  bulb  containing  mercury.  There  is  a  standard  mark 
on  the  stem,  at  the  top  of  which  is  a  pan. 


103] 


Specific  Gravity  of  Liquids. 


93 


The  weight  of  the  instrument  is  first  accurately  determined  in 
air  by  means  of  an  ordinary  balance.      Let  us  suppose  that   its 


Fig.  83. 

weight  is  618  grains,  and  that  the  liquid  whose  specific  gravity  is  to 
be  determined,  is  olive  oil.  The  hydrometer  is  placed  in  water,  and 
the  pan  loaded  with  weights,  until  the  liquid  is  level  with  the  mark 
on  the  stem.  Suppose  it  has 'been  necessary  to  add  93  grains  for 
this  purpose  ;  these  93  grains,  together  with  the  61 8  which  the  instru- 
ment weighs,  make  711  grains,  which  represents  the  weight  of 
water  displaced  by  the  instrument  (97).  The  hydrometer  is  then 
removed,  wiped  dry,  and  immersed  in  the  olive  oil.  Let  us  suppose 
that  now  only  3 1  grains  need  be  added  to  sink  the  hydrometer  to 
the  mark.  These  together  with  the  618  grains  which  the  instru- 
ment weighs,  in  all  649,  represent  the  weight  of  the  displaced  oil 
We  thus  learn  that  equal  volumes  of  oil  and  water  weigh  respec- 
tively 649  and  711.  Hence  we  obtain  the  specific  gravity  of 
the  latter  as  compared  with  the  former  by  dividing  649  by  711- 
The  quotient  is  0-91,  which  teaches  us  that  if  a  certain  volume 
of  water  weighs  100  grammes,  the  same  volume  of  oil  weighs  91 
grammes. 

Neither  Fahrenheit's  nor  Nicholson's  hydrometers,  give  such 
accurate  results  as  the  hydrostatic  balance. 

Specific  gravity  flask.     This  has  been  already  described.     In 


94  Hydrostatics.  [103- 

determining  the  specific  gravity  of  a  liquid,  the  flask  is  first  weighed, 
empty,  and  then,  successively,  full  of  water  and  of  the  given  liquid. 
If  the  weight  of  the  flask  be  subtracted  from  the  two  weights  thus 
obtained,  the  result  represents  the  weights  of  equal  volumes  of  the 
liquid,  and  of  water,  from  which  the  specific  gravity  is  obtained  by 
division. 

Specific  gravities  of  solids. 

Platinum  .'  «,'  .  .  22-069  Aluminum  .  .  .  2*68 
Gold  .  .  ,  \  19-36  Glass  ....  2-48 
Lead  .  .  .  .11-35  Anthracite  ,  .  .  i'8o 
Silver  ....  10-47  Coal  .  .  .  .  132 
Copper  .  .  .  8-87  Amber.  ,  .  ^.  1-07 
Iron  .  .  ,  .  778  Oak  ....  0-84 
Zinc  ....  6-86  Yellow  pine  .  .  .  0-65 
Diamonds  .  .  .  3-53  Common  poplar  .  .  0-38 
Statuary  marble  .  .  2-83  Cork  .  .  .  .  0-24 

Specific  gravities  of  liquids. 

Mercury       .         .         .  13-60  Distilled  water  at  o°  C.  0-99 

Bromine       .         .         .  2-96  Claret  .         .      "  »    ;    .  0-99 

Sulphuric  acid      .         .  1*84  Olive  oil        .        »"  .  0^91 

Milk     i        .         .         .  1*03  Oil  of  turpentine  .  .  0-87 

Sea  water     ,        .        .  1*02  Absolute  alcohol  J  .  0-80 

Distilled  water  at  4°  C.  i-oo  Ether   /.*''.-,•  .  072 

104.  Use  of  table*  of  specific  gravities. — Tables  of  specific 
gravity  admit  of  numerous  applications.  In  mineralogy  the  specific 
gravity  of  a  mineral  is  often  a  highly  distinctive  character.  Jewellers 
also  use  them.  By  means  of  tables  of  specific  gravities  the  weight 
of  a  body  may  be  calculated  when  its  volume  is  known,  and  con- 
versely the  volume  when  its  weight  is  known. 

With  a  view  to  explaining  the  last-mentioned  use  of  these  tables, 
it  will  be  well  to  explain  the  connection  existing  between  the  British 
units  of  length,  capacity,  and  weight.  It  will  be  sufficient  for  this 
purpose  to  define  that  which  exists  between  the  yard,  gallon,  and 
pound  avoirdtipois,  since  other  measures  stand  to  these  in  well- 
known  relations.  The  yard,  consisting  of  36  inches,  may  be 
regarded  as  the  primary  unit.  Though  it  is  essentially  an  arbitrary 
standard,  it  is  determined  by  this — that  the  simple  pendulum  which 
makes  one  oscillation  in  a  second,  at  London  on  the  sea  level, 
is  39' 1 3 75  inches  long  (61).  The  gallon  contains  277-274  cubic 


—106]  Beaumfs  Hydrometer.  95 

inches.  A  gallon  of  distilled  water  at  the  standard  temperature 
weighs  10  Ibs.  avoirdupois  or  70,000  grains  troy  ;  or,  which 
comes  to  the  same  thing,  one  cubic  inch  of  water  weighs  252-5 
grains. 

On  the  French  system  the  metre  is  the  primary  unit,  and  is  so 
chosen  that  10,000,000  metres  are  the  length  of  a  quadrant  of  the 
meridian  from  either  pole  to  the  equator.  The  metre  contains  10 
decimetres,  or  100  centimetres,  or  1,000  millimetres,  its  length 
equals  1-0936  yard.  The  unit  of  the  measure  of  capacity^  is 
the  litre  or  cubic  decimetre.  The  unit  of  weight  is  the  gramme, 
which  is  the  weight  of  a  cubic  centimetre  of  distilled  water  at  4°  C. 
The  kilogramme  contains  1,000  grammes,  or  is  the  weight  of  a 
decimetre  of  distilled  water  at  4°  C.  The  gramme  equals  15-443 
grains. 

Suppose  it  is  required  to  calculate  the  weight  of  a  cubic  foot  of 
coal.  A  cubic  foot  contains  1,728  cubic  inches  ;  the  weight  of  a 
cubic  foot  of  water  would  therefore  be  1,728  times  252-5  grains,  this 
being  the  weight  of  one  cubic  inch  of  water.  The  product  of  this 
multiplication  divided  by  7,000  grains  (the  number  contained  in  a 
pound  avoirdupois)  gives  62-3  pounds  as  the  weight  of  a  cubic  foot 
of  water  ;  and  as  we  learn,  from  the  tables,  that  coal  is  1-32  times 
as  heavy  as  water,  the  weight  of  a  cubic  foot  of  coal  will  be  1-32 
times  62-3  or  83-16  pounds. 

105.  Hydrometers  with  variable  volume. — The  hydrometers 
of  Nicholson  and  Fahrenheit  are  called  hydrometers  of  constant 
volume,  but  variable  weight,  because  they  are  always  immersed  to 
the  same  extent,  but  carry  different  weights.     There  are  also  hydro- 
meters of  variable  volume  but  of  constant  weight.     These  instru- 
ments known  under  the  different  names  of  acidometer,  alcoholometer, 
lactometer,  and  saccharometer,  are  not  used  to  determine  the  specific 
gravity  of  the  liquids,  but  to  show  whether  the  acids,  alcohols, 
solutions  of  sugar,  etc.,  are  more  or  less  concentrated. 

1 06.  Beaume's  hydrometer. — This,  which  was  the  first  of  these 
instruments,  may  serve  as  a  type  of  them.     It  consists  of  a  glass 
tube,  AB  (fig.  84),  loaded  at  its  lower  end  with  mercury,  and  with 
a  bulb  blown  in  the  middle.     The  stem,  the  external  diameter  of 
which  is  as  regular  as  possible,  is  hollow,  and  the  scale  is  marked 
upon  it. 

The  graduation  of  the  instrument  differs  according  as  the  liquid, 
for  which  it  is  to  be  used,  is  heavier  or  lighter  than  water.  In  the 
first  case  it  is  so  constructed,  that  it  sinks  in  water  nearly  to  the 


Hydrostatics. 


[106- 


•Fig.  84. 


top  of  the  stem,  to  a  point  A,  which  is  marked  zero.  A  solution  of 
fifteen  parts  of  salt  in  eighty-five  parts  of  water  is  made,  and  the 
instrument  immersed  in  it.  It  sinks  to  a 
certain  point  on  the  stem,  B,  which  is 
marked  15  ;  the  distance  between  A  and  B 
is  divided  into  15  equal  parts,  and  the 
graduation  continued  to  the  bottom  of  the 
stem.  Sometimes  the  graduation  is  on  a 
piece  of  paper  in  the  interior  of  the  stem. 

The  hydrometer  thus  graduated  only  serves 
for  liquids  of  a  greater  specific  gravity  than 
water,  such  as  acids  and  saline  solutions. 
For  liquids  lighter  than  water  a  different 
plan  must  be  adopted.  Beaume  took  for 
zero  the  point  to  which  the  apparatus  sank 
in  a  solution  of  10  parts  of  salt  in  90  of 
water,  and  for  10°  he  took  the  level  in 
distilled  water.  This  distance  he  divided 
into  10°,  and  continued  the  division  to  the  top  of  the  scale. 

The  graduation  of  these  hydrometers  is  entirely  arbitrary,  and 
they  give  neither  the  densities  of  the  liquids,  nor  the  quantities 
dissolved.  But  they  are  very  useful  in  making  mixtures  or  solu- 
tions in  given  proportions  ;  the  results  they  give  being  sufficiently 
near  in  the  majority  of  cases.  For  instance,  it  is  found  that  a 
well-made  syrup  marks  35°  on  Beaume"'s  hydrometer,  from  which 
a  manufacturer  can  readily  judge  whether  a  syrup  which  is  being 
evaporated  has  reached  the  proper  degree  of  concentration. 

107.  Gay-Xiussac's  alcoholometer — The  spirits  of  wine  and 
brandy,  in  daily  use,  are  a  mixture  of  pure  alcohol  and  water.  The 
more  alcohol  they  contain  the  stronger  they  are ;  the  more  water 
they  contain  so  much  the  weaker  are  they.  Hence  it  is  important 
to  have  a  simple  means  of  exactly  determining  the  quantity  of  water 
contained  in  spirituous  liquors.  This  is  effected  by  means  of  Gay- 
Lussac's  alcoholometer,  which  has  the  same  shape  as  Beaume's, 
and  only  differs  in  the  graduation.  This  is  effected  as  follows  : — 

Mixtures  of  absolute  alcohol  and  distilled  water  are  made,  con- 
taining 5,  10,  20,  30,  etc.,  per  cent,  of  the  former.  The  alcoholo- 
meter is  so  constructed  that  when  placed  in  pure  distilled  water, 
the  bottom  of  its  stem  is  level  with  the  water,  and  this  point  is  zero. 
It  is  next  placed  in  absolute  alcohol,  which  marks  100°,  and  then 
successively  in  mixtures  of  different  strengths,  containing  10,  20, 


-108] 


Lactometer. 


97 


30,  etc.,,  per  cent.      Ths  divisions  thus  obtained  are  not  exactly 

equal,  but  their  difference  is  not  great,  and  they  are  subdivided  into 

ten  divisions,  each  of  which  marks  one  per  cent. 

of  absolute  alcohol  in  a  liquid.     Thus  a  brandy 

in  which  the  alcoholometer  stood  at  48,  would 

contain  48  per  cent,  of  absolute   alcohol,  and 

the  rest  would  be  water. 

All  these  determinations  are  made  at  15°  C., 
and  for  that  temperature  only  are  the  indications 
correct.  For,  other  things  being  the  same,  if  the 
temperature  rises  the  liquid  expands,  and  the 
alcoholometer  will  sink,  and  the  contrary,  if  the 
temperature  falls.  To  obviate  this  error  Gay- 
Lussac  constructed  a  table  which  for  each 
percentage  of  alcohol  gives  the  reading  of  the 
instrument  for  each  degree  of  temperature  from 
o°  up  to  30°.  When  the  exact  analysis  of  an 
alcoholic  mixture  is  to  be  made,  the  temperature 
of  the  liquid  is  first  determined,  and  then  the 
point  to  which  the  alcoholometer  sinks  in  it.  The  number  in  the 
table  corresponding  to  these  data  indicates  the  percentage  of 
alcohol.  From  its  giving  the  percentage  of  alcohol,  this  is  often 
called  the  centesimal  alcoholometer. 

1 08.  lactometer.  —  The  lactometer  is  a  hydrometer  like 
Beaume's,  specially  graduated  for  the  purpose  of  ascertaining  the 
quality  of  milk  (fig.  86).  This  is  accom- 
plished in  the  following  manner  : — The 
instrument  is  immersed  in  a  vessel 
containing  pure  milk,  and  the  point 
to  which  it  sinks  is  marked  zero  on  a 
paper  strip  affixed  to  the  stem.  Mix- 
tures are  then  made  of  ^  of  milk  and 
^  of  water  ;  of  ^  and  T25,  and  so  on  to 


Fig.  85. 


—  of  milk  and  ~  of  water.     The  lac- 


Fig.  86. 


tometer  is  successively  immersed  in 
these,  and  sinks  to  different  depths  ;  the  point  at  which  it  stops  in 
each  case  is  marked  by  a  number  on  the  stem,  and  thus  indicates 
a  milk  of  a  particular  strength,  that  is,  one  containing  a  certain 
quantity  of  admixed  water. 

The  lactometer  is,  however,  no  infallible  test  for  the  adulteration 
of  milk  ;  for  the  density  of  natural  milk  is  subject  to  variation,  and 
an  apparent  fraud  may  really  be  due  to  a  bad  natural  quality  of  milk. 


98  On  Gases.  [109- 


B  O  O  K     III. 

ON   GASES. 
CHAPTER   I. 

PROPERTIES   OF  GASES.      ATMOSPHERE.      BAROMETERS. 

109.  Physical  properties  of  gases. — Gases,  as  we  have  already 
seen,  are  bodies  whose  molecules  are  in  a  constant  state  of  repul- 
sion in  virtue  of  which  they  possess  the  most  perfect  mobility,  and 
are  continually  tending  to  occupy  a  greater  space.  This  property 
of  gases  is  known  by  the  names  expansibility,  tension,  or  elastic 
force,  from  which  they  are  often  called  elastic  fluids. 

The  number  of  gases  with  which  chemistry  makes  us  acquainted 
is  very  considerable ;  but  only  four  are  elementary ;  these  are  oxygen, 
hydrogen,  nitrogen,  and  chlorine.  Some  gases  are  coloured,  but 
most  of  them  are  colourless.  Some  have  a  disagreeable  odour, 
others  are  quite  inodorous.  Some  are  noxious,  acting  as  poison 
to  men  and  animals  which  breathe  them  ;  such  are  carbonic  oxide, 
which  is  produced  by  the  combustion  of  charcoal ;  sulphuretted 
hydrogen,  which  is  given  off  from  drains.  Others  are  inoffensive, 
such  as  nitrogen  and  hydrogen  ;  yet  an  animal  cannot  live  in  them. 
They  are  not  deleterious,  in  the  sense  of  being  poisonous  ;  but  they 
do  not  support  life.  The  only  gas  which  has  this  property  is  oxy- 
gen; an  animal  deprived  of  this  gas,  even  for  a  few  seconds,  soon  dies. 

Gases  and  liquids  have  several  properties  in  common,  and  some 
in  which  they  seem  to  differ  are  in  reality  only  different  degrees 
of  the  same  property.  Thus,  in  both,  the  particles  are  capable  ot 
moving  ;  in  gases  quite  freely  ;  in  liquids  not  quite  freely,  owing 
to  a  certain  degree  of  viscosity.  Both  are  compressible,  though 
in  very  different  degrees  ;  if  a  liquid  and  a  gas  both  exist  under 
a  pressure  of  one  atmosphere,  and  then  the  pressure  be  doubled, 
the  water  is  compressed  by  about  the  200*600^  Part  (75)?  while  the 
gas  is  compressed  by  one-half.  In  density  there  is  a  great  difference  ; 
water,  which  is  the  type  of  liquids,  is  about  770  times  as  heavy  as 


-110]  Atmospheric  Air.  99 

air,  the  type  of  gaseous  bodies,  while  under  a  pressure  ot  one 
atmosphere.  The  property  by  which  gases  are  distinguished  from 
liquids  is  their  tendency  to  indefinite  expansion. 

Matter  assumes  the  solid,  liquid,  or  gaseous  form  according  to 
the  relative  strength  of  the  cohesive  and  repulsive  forces  exerted 
between  their  particles.  In  liquids  these  forces  balance  ;  in  gases 
repulsion  preponderates. 

By  the  aid  of  pressure  and  of  very  low  temperatures,  the  force 
of  cohesion  may  be  so  far  increased  in  many  gases  that  they  are 
converted  into  liquids ;  and  there  is  reason  for  believing  that,  with  a 
sufficient  degree  of  pressure  and  cold,  they  might  all  be  liquefied.  On 
the  other  hand,  heat,  which  increases  the  force  of  repulsion,  converts 
liquids,  such  as  water,  alcohol,  and  ether,  into  the  aSriform  state  in 
which  they  obey  all  the  laws  of  gases.  This  ae'riform  state  of 
liquids  is  known  by  the  name  of  vapour,  while  gases  are  bodies 
which  under  ordinary  temperature  and  pressure,  remain  in  the 
aeriform  state. 

In  describing  the  properties  of  gases  we  shall  for  obvious  reasons, 
have  exclusive  reference  to  atmospheric  air  as  their  type. 

no.  Atmospheric  air. — Air  is  the  gaseous  fluid  in  which  wev 
live.  It  was  regarded  by  the  ancients  as  one  of  the  four  elements. 
Modern  chemistry,  however,  has  shown  that  it  is  a  mixture  of 
oxygen  and  nitrogen  gases  in  the  proportion  of  2O'8  volumes  of 
the  former  to  79^2  volumes  of  the  latter.  By  weight  it  consists  of 
23  parts  of  oxygen  to  77  parts  of  nitrogen. 

The  oxygen  feeds  all  the  combustions,  which  are  produced  round 
about  us  ;  and  it  also  supports  animal  life.  If  it  alone  were  present, 
or  even  if  it  were  present  in  a  larger  proportion,  the  combustions 
would  be  too  brisk,  and  life  too  active.  The  coal  of  our  fireplaces 
would  burn  almost  instantaneously,  and  even  the  grates  in  which 
it  is  contained  would  take  fire.  Life  would  be  promptly  destroyed 
by  so  active  an  agent.  The  function  of  the  nitrogen  is  to  attenuate 
the  too  powerful  effects  of  the  oxygen. 

Air  is  inodorous,  transparent,  and  colourless,  at  any  rate  in  small 
masses.  In  larger  masses  it  is  blue  ;  thus  arises  the  blue  colour  of 
the  sky.  Without  air  the  celestial  vault  would  appear  black  ;  it 
appears  almost  so  when  viewed  from  the  tops  of  very  high  mountains, 
and  from  balloons  ;  for  then  the  air  above  is  very  highly  rarefied. 

Air  too,  in  virtue  of  its  elasticity,  is  the  medium  for  transmitting 
sounds  ;  so  that,  if  we  were  without  it,  the  use  of  speech  and  of 
music  would  be  lost. 

H  2 


100 


On  Gases. 


[Ill- 


Fig.  87. 


ill.  Expansibility  of  gases. — This  property  of  gases,  their 
tendency  to  assume  continually  a  greater  volume,  is  exhibited  by 
means  of  the  following  experiment.  A  bladder  closed  by  a  stop- 
cock, moistened  so  as  to  render  it  more  flexible,  and  about  half  full 
of  air,  is  placed  under  the  receiver  of  the  air 
pump  (rig.  87),  and  a  partial  vacuum  is  produced, 
on  which  the  bladder  immediately  distends. 
This  arises  from  the  fact  that  the  molecules 
of  air  repel  each  other,  and  press  against  the 
sides  of  the  bladder.  Under  ordinary  con- 
ditions this  internal  pressure  is  counter- 
balanced by  the  air  in  the  receiver,  which 
exerts  an  equal  and  contrary  pressure.  But 
when  this  pressure  is  removed  by  exhausting 
the  receiver,  the  internal  pressure  becomes 
evident.  When  air  is  again  admitted  into  the 
receiver  the  bladder  resumes  its  original  form. 
The  same  effects  would  be  produced  what- 
ever gases  were  contained  in  the  bladder,  thus  showing  that  all  are 
expansible. 

112.  "Weight  of  gases. — From  their  extreme 
fluidity  and  expansibility,  gases  seem  to  be  unin- 
fluenced by  the  force  of  gravity  ;  they  nevertheless 
possess  weight,  like  solids  and  liquids.  To  show 
this,  a  glass  globe  of  3  or  4  quarts  capacity  is 
taken  (fig.  88),  the  neck  of  which  is  provided  with  a 
stop-cock,  which  hermetically  closes  it,  and  by 
which  it  can  be  screwed  to  the  plate  of  the  air 
pump.  The  globe  is  then  completely  exhausted,  and 
its  weight  determined  by  means  of  a  delicate  balance. 
Air  is  now  allowed  to  enter,  and  the  globe  again 
weighed.  The  weight  in  the  second  case  will  be 
found  to  be  greater  than  before,  and  if  the  capa- 
city of  the  vessel  is  known,  the  increase  will  ob- 
viously be  the  weight  of  that  volume  of  air. 

By  a  modification  of  this  method,  and  with  the 
adoption  of  certain  precautions,  the  weight  of  air 
and  of  other  gases  has  been  determined  :  100 
cubic  inches  of  dry  air  under  the  ordinary  atmospheric  pressure  of 
30  in.  and  at  the  temperature  of  16°  C.,  weigh  31  grains  ;  the  same 
volume  of  carbonic  acid  gas  under  the  same  circumstances  weighs 


Fig.  88. 


- 114]  A  tmospheric  Pressure.  i  o  I 

47-25  grains ;  100  cubic  inches  of  hydrogen,  the  lightest  of  all  gases, 
weigh  2-14  grains;  and  100  cubic  inches  of  hydriodic  acid  gas 
weigh  146  grains. 

The  ratio  of  the  destiny  of  air  at  o°  C.  and  30  inches  pressure 
to  that  of  water  at  o°  C.  is  found  to  be  O'ooi296.  In  other  words, 
the  latter  is  about  770  times  as  heavy  as  the  former. 

113.  The  atmosphere.  Experiments  proving  its  weight. — 
The  atmosphere  is  the  name  given  to  the  layer  of  air  which,  like 
a  light  coating,  surrounds  our  globe  in  every  part.  It  shares  the 
rotatory  motion  of  the  globe,  and  would  remain  fixed  relatively  to 
terrestrial  objects,  but  for  local  circumstances,  which  produce  winds, 
and  are  constantly  disturbing  its  equilibrium. 

The  existence  of  this  gaseous  mass  is  proved  by  the  winds,  which 
incessantly  blow  on  the  surface  of  the  earth  ;  by  the  flight  of  birds, 
and  the  suspension  of  clouds. 

Besides  the  oxygen  and  nitrogen  of  which  the  air  is  composed, 
it  also  contains  a  quantity  of  aqueous  vapour,  which  varies  with  the 
temperature,  the  season,  the  locality,  and  the  direction  of  the  winds. 
The  mean  amount  of  this  in  London  is  from  5  to  6  grains  in  a  cubic 
foot  of  air. 

It  further  contains  from  3  to  6  parts  in  10,000  of  carbonic  acid. 
This  arises  from  the  respiration  of  man  and  animals,  from  the  decay 
of  organic  matter,  and  from  the  combustion  of  wood  and  coal.  ' 
This  latter  cause  of  the  production  of  carbonic  acid  increases  every 
year.  It  has  been  calculated  that  in  Europe  alone  about  ioj.  mil- 
liards of  cubic  yards  of  carbonic  acid  are  every  year  sent  into  the 
atmosphere  from  this  source.  This  mass  of  gas  is  equal  to  what 
would  be  produced  by  509  millions  of  individuals,  each  by  the  act 
of  respiration  converting  1 54  grains  of  carbon  in  the  system  into 
carbonic  acid  every  hour. 

Notwithstanding  this  enormous  continual  production  of  carbonic 
acid  on  the  surface  of  the  globe,  the  composition  of  the  atmosphere 
does  not  vary ;  for  plants  in  the  process  of  vegetation  decompose 
the  carbonic  acid,  assimilating  the  carbon,  and  restoring  to  the 
atmosphere  the  oxygen  which  is  being  continually,  consumed  in  the 
processes  of  respiration  and  combustion. 

Thus,  by  a  natural  harmony,  the  atmosphere  retains  an  almost 
uniform  quantity  of  this  gas,  so  that  there  is  no  fear  of  its  accu- 
mulating to  such  an  extent  as  to  be  injurious  to  the  human  species. 
1 14.  Atmospheric  pressure. — Having  seen  that  air  has  weight, 
it  is  easy  to  conceive  that  the  great  mass  of  air  which  constitutes 


1O2 


On  Gases. 


[114- 


the  atmosphere  must  exert  a  great  pressure  on  the  surface  of  the 
earth,  arid  on  all  bodies  found  there.  This  pressure  is  called 
the  atmospheric  pressure.  It  necessarily  decreases  as  we  ascend  in 
the  atmosphere ;  for  if  we  conceive  the  atmosphere  resolved  into 
horizontal  layers  superposed  on  each  other,  it  is  clear  that  the 
lower  layers  which  support  the  weight  of  the  whole  atmosphere  are 
the  most  compressed,  and  the  most  dense  ;  while  the  higher  layers 
are  less  and  less  compressed,  and  therefore  less  and  less  dense. 
This  is  expressed  by  saying  that  they  are  more  rarefied  or  more 
rare.  In  saying  that  100  cubic  inches  of  air  weighed  31  grains,  it 
was  understood  that  air  at  the  sea  level  was  referred  to  ;  at  any 
greater  height  this  volume  of  air' would  weigh  less. 

The  pressure  of  the  atmosphere  may  be  demonstrated  by  a 
number  of  experiments,  among  which  are  the  following  : 

115.  Crushing  force  of  the  atmosphere.— On  one  end  of  a 
stout  glass  cylinder,  about  5  inches  high,  and  open  at  both  ends,  a 


Fig.  90- 

piece  of  bladder  is  tied  quite  air-tight.  The  other  end,  the  edge  of 
which  is  ground  and  well  greased,  is  pressed  on  the  plate  of  the  air- 
pump  (fig.  89).  The  bladder  is  pressed  downwards  by  the  weight 
of  the  atmosphere,  and  is  pressed  upwards  by  the  expansive  force  of 
the  air  in  the  cylinder.  These  two  pressures  at  first  counterbalance 
each  other ;  the  bladder  is  not  pressed  in  either  direction,  but  as 
soon  as ;  the  internal  air  is  removed  from  the  vessel,  by  working 
the  air-pump,  the  bladder  is  depressed  by  the  weight  of  the  atmo- 


116] 


Magdeburg  HcmispJieres. 


103 


sphere  above  it,  and  finally  bursts  with  a  loud  report  caused  by  the 
sudden  entrance  of  the  air. 

116.  Magdeburg;  hemispheres. — The  preceding  experiment 
only  serves  to  illustrate  the  downward  pressure  of  the  atmosphere. 
By  means  of  the  Magdeburg  hemispheres  (fig.  90),  the  invention  of 
which  is  due  to  Otto  von  Guericke,  burgomaster  of  Magdeburg,  it 
can  be  shown  that  the  pressure  acts  in  all  directions.  This  ap- 
paratus consists  of  two  hollow  brass  hemispheres  of  4  to  4^  inches 


Fig.  91. 

diameter,  the  edges  of  which  are  made  to  fit  tightly,  and  are  well 
greased.  One  of  the  hemispheres  is  provided  with  a  stopcock,  by 
which  it  can  be  screwed  on  the  air-pump,  and  on  the  other  there  is 
a  handle.  As  long  as  the  hemispheres  contain  air  they  can  be 
separated  without  any  difficulty,  for  the  external  pressure  of  the 
atmosphere  is  counterbalanced  by  the  elastic  force  of  the  air  in 
the  interior.  But  when  the  air  in  the  interior  is  pumped  out  by 
means  of  the  air-pump,  the  hemispheres  cannot  be  separated  with- 
out a  powerful  effort,  fig.  91  ;  and  as  this  is  the  case  in  whatever 
position  they  are  held,  it  follows  that  the  atmospheric  pressure  is 
transmitted  in  all  directions. 

We  shall  presently  see  (119)  that  the  pressure  of  the  atmosphere 


104 


On  Gases. 


[116 


on  a  square  inch  is  about  15  Ibs.  Hence  if  in  the  above  experiment, 
the  area,  not  of  each  of  the  hemispheres,  but  of  the  circle  along  which 
they  are  pressed,  is  10  square  inches,  the  force  by  which  they  are 
pressed  together  is  150  Ibs.  and  this  force  would  be  required  to 
separate  them. 

It  is  related  that  Otto  von  Guericke,  the  inventor  of  this  appa- 
ratus, constructed  hemispheres  the  internal  diameter  of  which  was 
about  2  feet ;  when  applied  against  each  other  and  exhausted,  twelve 
horses,  six  pulling  at  each  hemisphere,  were  required  to  separate 
them. 

DETERMINATION   OF  THE  ATMOSPHERIC  PRESSURE. 
BAROMETERS. 


117.  Torricelli's  experi- 
ment.— The  above  experiments 
demonstrate  the  existence  of  the 
atmospheric  pressure,  but  they 
give  no  indications  as  to  its 
amount.  The  following  experi- 
ment, which  was  first  made  in 
1643  by  Torricelli,  a  pupil  of 
Galileo,  not  merely  proves  the 
pressure  of  the  atmosphere,  but 
also  gives  an  exact  measure  of 
its  weight. 

A  glass  tube  is  taken,  about 
a  yard  long,  and  a  quarter  of  an 
inch  internal  diameter  (fig.  92). 
It  is  sealed  at  one  end,  and  is 
quite  filled  with  mercury.  The 
aperture  C  being  closed  by  the 
thumb,  the  tube  is  inverted,  the 
open  end  placed  in  a  small 
mercury  trough,  and  the  thumb 
removed.  The  tube  being  in  a 
vertical  position,  the  column  of 
mercury  sinks,  and  after  oscil- 
lating some  time,  it  finally  comes 
to  rest  at  a  height  A,  which  at 


Fig.  92. 


the  level  of  the  sea  is  about  thirty  inches  above  the  mercury  in 
the  trough.     The  mercury  is  raised  in  the  tube  by  the  pressure  of 


-119]         A  mount  of  the  A  tmospheric  Pressure  105 

the  atmosphere  on  the  mercury  in  the  trough.  There  is  no  contrary 
pressure  on  the  mercury  in  the  tube,  because  it  is  closed.  But  if 
the  end  of  the  tube  be  opened,  the  atmosphere  will  press  equally 
inside  and  outside  the  tube,  and  the  mercury  will  sink  to  the  level 
of  that  in  the  trough.  It  has  been  shown  in  hydrostatics  (88)  that 
the  heights  of  two  columns  of  liquid  in  communication  with  each 
other  are  inversely  as  their  densities  ;  and  hence  it  follows,  that 
the  pressure  of  the  atmosphere  is  equal  to  that  of  a  column  of  mer- 
cury, the. height  of  which  is  thirty  inches.  That  the  mercury  sank 
in  the  first  case  was  due  to  its  weight  being  greater  than  the  pressure 
of  the  atmosphere.  If,  however,  the  weight  of  the  atmosphere 
diminishes,  the  height  of  the  column  which  it  can  sustain  must  also 
diminish. 

1 1 8.  Pascal's  experiments. — Pascal,  who  wished  to  prove  that 
the  force  which  sustained  the  mercury  in  the  tube  was  really  the 
pressure  of  the  atmosphere,  made  the  following  experiments  : — i.  If 
it  were  the  case,  the  column  of  mercury  ought  to  descend  in  propor- 
tion as  we  ascend  in  the  atmosphere  (i  14).   He  accordingly  requested 
one  of  his  relations  to  repeat  Torricelli's  experiment  on  the  summit 
of  the  Puy  de  Dome  in  Auvergne.     This  was  done,  and  it  was  found 
that  the  mercurial  column   was   about   three   inches  lower,  thus 
proving  that  it  is  really  the  weight  of  the  atmosphere  which  supports 
the  mercury,  since,  when  this  weight  diminishes,  the  height  of  the 
column  also  diminishes,    ii.  Pascal  repeated  Torricelli's  experiment 
at  Rouen,  in  1646,  with  other  liquids.     He  took  a  tube  closed  at 
one  end,  nearly  40  feet  long,  and  having  filled  it  with  water,  placed 
it  vertically  in  a  vessel  of  water,  and  found  that  the  water  stood  in 
the  tube  at  a  height  of  34  feet ;   that  is,  1 3-6  times  as  high  as 
mercury.     But  since  mercury  is   13-6  times  as  heavy  as  water,  the 
weight  of  the  column  of  water  was  exactly  equal  to  that  of  the 
column   of  mercury  in  Torricelli's   experiment,  and   it   was  con- 
sequently the  same  force,  the  pressure  of  the  atmosphere,  which 
successively  supported  the  two  liquids.     Pascal's  other  experiments 
with  oil  and  with  wine  gave  similar  results.     He  found,  for  in- 
stance that  a  column  of  oil  stood  at  a  height  of  about  37  feet. 

119.  Amount  of  the  atmospheric  pressure. — Let  us  assume 
that  the  tube  in  the  above  experiment  is  a  cylinder,  the  cross-section 
of  which  is  equal  to  a  square  inch,  then,  since  the  height  of  the 
mercurial  column  in  round  numbers  is  30  inches,  the  column  will 
contain  30  cubic  inches,  and  as  a  cubic  inch  of  mercury  weighs 
3433-5  grains  =  0-49  of  a  pound,  the  pressure  of  such  a  column  on  a 


io6 


On  Gases. 


[119- 


square  inch  of  surface  is  equal  to  14*7  pounds.  In  round  numbers 
the  pressure  of  the  atmosphere  is  taken  at  15  pounds  on  the  square 
inch.  A  surface  of  a  foot  square  contains  144  square  inches,  and 
therefore  the  pressure  upon  it  is  equal  to  2,160  pounds,  or  nearly  a 

ton. 

A  gas  or  a  liquid  which  acts  in 
such  a  manner  that  a  square  inch 
of  surface  is  exposed  to  a  pressure, 
1 5  pounds,  is  said  to  exert  a  pressure 
of  one  atmospliere.  If,  for  instance, 
the  elastic  force  of  the  steam  of  a 
boiler  is  so  great  that  each  square 
inch  of  the  internal  surface  is  ex- 
posed to  a  pressure  of  90  pounds 
(  =  6x  15),  we  say  it  was  under  a 
pressure  of  six  atmospheres. 

120.  Different  kinds  of  baro- 
meters. —  The   instruments    used 
for     measuring     the     atmospheric 
pressure  are  called  barometers,  from 
two    Greek   words    which    signify 
measure  of  weight  (air,  of  course, 
being  understood).    In  ordinary  ba- 
rometers, the  pressure* is  measured 
by  the  height  of  a  column  of  mer- 
cury, as  in  Torricelli's  experiment  ; 
the     barometers     which     we     are 
about  to  describe  are  of  this  kind. 
But  there  are  barometers  without 
mercury,  one  of  which,  the  aneroid 
(135)  is  remarkable  for  its  simplicity 
and  portability. 

121.  Cistern  barometer. — Or- 
dinary barometers   are  classed  as 
syphon    and     cistern     barometers. 
Fig.  93  represents  the  usual  form  of 
the  cistern  barometer.     It  consists 
of  a  glass  tube  ai,  closed  at  one 
end,  about  thirty-three  inches  long, 

Fig.  93.  and  about  half  an  inch  in  diameter. 

The  tube  is  filled  with  mercury,  and  then  its  open  end  is  inverted 


-122]  Fortiris  Barometer.  107 

in  mercury  contained  in  a  glass  vessel,  A,  of  a  peculiar  shape  ; 
only  the  front  half  of  this  is  visible,  the  other  being  fixed  in  a 
mahogany  board  which  supports  the  whole  barometer.  The  bottom 
of  the  cistern  forms  a  spherical  well,  which  is  filled  with  mercury, 
and  in  which  the  tube  ai  is  immersed.  The  tube  is  not  fixed  tightly 
in  the  neck,  so  that  the  atmospheric  pressure  can  be  freely  trans- 
mitted to  the  mercury  of  the  bath,  and  thus  supports  the  column  of 
mercury  ai.  If  the  pressure  increases  the  mercury  rises,  if  it  de- 
creases the  mercury  sinks. 

At  the  top  of  the  tube  on  the  right  is  a  scale  divided  in  inches 
to  measure  the  height  of  the  mercury  in  the  tube.  The  graduation 
starts  from  the  zero  which  is  level  with  the  mercury  in  the  bath. 
Hence,  if  the  top  of  the  mercury  at  a  stands  at  thirty  inches,  for 
instance,  this  signifies  that  the  height  of  the  column  of  mercury  is 
thirty  inches.  Only  a  portion  of  the  scale  is  given,  since,  for  ordi- 
nary purposes,  the  variations  of  the  atmospheric  pressure  are  within 
a  few  inches.  Where  greater  variations  occur,  as  in  the  use  of  the 
barometer  for  measuring  heights,  the  graduated  part  must  be 
longer. 

It  will  be  observed  that  the  starting-point  of  the  graduation,  the 
zero,  is  at  the  level  of  the  mercury  in  the  cistern.  But  the  zero  of 
the  scale  does  not  always  correspond  to  the  level  of  the  mercury  in 
the  cistern.  For  as  the  atmospheric  pressure  is  not  always  the 
same,  the  height  of  the  mercurial  column  varies  ;  sometimes 
mercury  is  forced  from  the  cistern  into  the  tube,  and  sometimes 
from  the  tube  into  the  cistern,  so  that,  in  the  majority  of  cases,  the 
graduation  of  the  barometer  does  not  indicate  the  true  height.  To 
diminish  this  source  of  error,  the  cistern  has  the  form  represented  in 
fig.  93.  Its  upper  part,  that  corresponding  to  the  level  of  the 
mercury,  is  about  four  inches  in  diameter  ;  so  that,  whether  the 
mercury  passes  from  the  cistern  into  the  tube,  or  from  the  tube  into 
the  cistern,  as  it  is  spread  over  a  large  surface  the  variations  in  the 
level  are  very  small  and  may  be  neglected. 

To  complete  this  description  it  may  be  added,  that  on  the 
scale  is  a  small  index,  c,  sliding  along  a  vertical  rod.  When 
made  level  with  the  mercury  this  index  points  on  the  one  side  to 
the  divisions  on  the  graduated  scale,  and,  on  the  other  side,  to 
certain  inscriptions,  the  use  of  which  will  be  afterwards  stated  (127). 
Lastly,  in  the  middle  of  the  tube  are  two  thermometers,  one  with  a 
Fahrenheit  and  the  other  with  a  Centigrade  graduation. 

122.  Fox-tin's  barometer. — Forties  barometer  (fig.  94)  differs 


io8 


On  Gases. 


[122- 


from  that  just  described,  in  the  shape  of  the  cistern.  The  base 
of  the  cistern  is  made  of  leather,  and  can  be  raised  or  lowered 
by  means  of  a  screw ;  this  has  the  advantage,  that  a  constant 
level  can  be  obtained,  and  also  that  the  instrument  is  made  more 

portable.     For,  in  travelling,  it  is 

only  necessary  to  raise  the  leather 

until  the  mercury,  which  rises  with 

it,  quite  fills  the  cistern  ;  the  ba- 
rometer may  then  be  inclined,  and 

even  inverted,  without  any  fear  that 

a  bubble  of  air  may  enter,  or  that 

the  shock  of  the  mercury  may  crack 

the  tube. 

Fig.  95  shows  the  construction 

of  the  cistern.    It  consists  of  a  glass 

cylinder,  &,  which  allows  the  mer- 
cury to'be  seen  ;  the  bottom  of  the 

cylinder  is  cemented  to  a  box-wood 

cylinder,  zz,  on  which  is  firmly  fixed 

at  ii the  chamois  leather,  inn,  which 

is  the  base  of  the  cistern.     At  the 

bottom  of  this  leather  is   a   small 

wooden  button,  r,  against  which  the 

screw  C  works,  by  which  it  is  raised 

or  lowered.     This  screw  works  in 

the  bottom   of    a   brass    cylinder, 

G,  which  is  fastened  on  the  glass 

cylinder.     At  the  top  of  the  cistern 

there  is  a  small  ivory  pointer,  a, 

the  point   of    which    exactly   cor- 
responds to  the  zero  on  the  scale. 

The  upper  part   of  the   cistern  is 

closed  by  buckskin,  ce,  which  is  fastened  to  the  barometer 

tube,  E,  and  to  a  tubulure  in  the  wooden  disc,  which 
Fig.  94.  covers  the  cistern.  The  barometer  tube  is  drawn  out 
at  the  open  end,  which  is  immersed  in  the  mercury.  The  atmo- 
spheric pressure  is  transmitted  through  the  pores  of  the  leather. 
In  using  this  barometer,  the  mercury  is  first  made  level  with 
the  point  a,  which  is  effected  by  turning  the  screw  C  either  in 
one  direction  or  the  other.  In  this  manner  the  distance  of  the 
top,  B,  of  the  column  of  mercury  from  the  ivory  point  a,  gives 


Fig.  95- 


-123] 


Gay-Lussacs  Syphon  Barometer. 


109 


, 


exactly  the  height  of  the  barometer.  For  the  graduation  is 
measured  from  the  point  a.  Lastly,  the  lower  part  of  the  cistern 
is  enclosed  in  a  brass  case,  which  is  connected  with  the  lid  by 
three  screws  k,  k,  k.  To  the  cistern  is  screwed  a  long  brass  case, 
which  encloses  the  whole  of  the  tube,  as  seen  in  figure.  At  the  top 
of  this  case  there  are  two  longitudinal  slits,  on  opposite  sides, 
so  that  the  level  of  the  mercury,  B,  is  seen.  The  scale  on  the 
case  is  graduated  in  millimetres  or  in  inches.  An  index,  A, 
moved  by  the  hand,  gives,  by 
means  of  a  vernier,  the  height  j 
of  the  mercury  to  ~  of  a  milli- 
metre. At  the  bottom  of  the 
case  is  affixed  a  thermometer  to 
indicate  the  temperature. 

123.  Gay-Xrtissac's  syphon 
barometer.  —  The  syphon  baro- 
meter has  no  cistern,  but  con- 
sists of  a  bent  glass  tube  (fig. 
96),  one  of  the  branches  of  which 
is  much  longer  than  the  other. 
The  longer  branch,  which  is 
closed  at  the  top,  is  filled  with 
mercury  as  in  the  cistern  baro- 
meter, while  the  shorter  branch, 
which  is  open,  serves  as  a 
cistern.  The  difference  between 
the  two  levels  is  the  height  of 
the  barometer. 

Fig.  96  represents  the  syphon 
barometer  as  modified  by  Gay- 
Lussac.  In  order  to  render  it 
more  available  for  travelling,  -by 

preventing  the  entrance  of  air, 
I    .  .      ,  &,  ' 

he  joined  the  two  branches  by  a 

capillary  tube  ;  when  the  instrument  is  inverted  (fig.  97),  the  tube 
always  remains  full  in  virtue  of  its  capillarity,  and  air  cannot  pene- 
trate into  the  longer  branch,  which,  of  course,  is  absolutely  necessary. 
A  sudden  shock,  however,  might  separate  the  mercury  and  admit 
some  air.  To  avoid  this  M.  Bunten  has  introduced  an  ingenious 
modification  into  the  apparatus.  The  longer  branch,  A,  is  drawn 
out  to  a  fine  point,  and  is  joined  to  a  tube,  B,  of  the  form  repre- 


.  96. 


Fig.  97. 


Fig.  98. 


IIO 


On  Gases. 


[123- 


sented  in  fig.  98.  By  this  arrangement,  if  air  passes  through  the 
capillary  tube,  it  cannot  penetrate  the  drawn-out  extremity  of  the 
longer  branch,  but  lodges  in  the  upper  part  of  the  enlargement  B. 
Jn  this  position  it  does  not  affect  the  observations,  since  the  vacuum 
is  always  at  the  upper  part  of  the  tube  ;  it 
is,  moreover,  easily  removed. 

In  Gay-Lussac's  barometer  the  shorter 
branch  is  closed,  but  there  is  a  lateral 
capillary  aperture  *,  through  which  the  at- 
mospheric pressure  is  transmitted. 

The  barometric  height  is  determined  by 
means  of  two  scales,  which  have  a  common 
zero  at  the  middle  of  the  longer  branch,  and 
are  graduated  in  contrary  directions,  the  one 
from  the  middle  to  a,  and  the  other  from 
the  middle  to  b,  either  on  the  tube  itself,  or 
on  brass  rules  fixed  parallel  to  the  tube. 
Two  sliding  indexes  are  moved  until  they 
correspond  to  the  level  of  the  mercury  in  a 
and  b.  The  total  height  of  the  barometer 
ab  is  the  sum  of  the  distances  from  the 
middle  to  a  and  b  respectively. 

124.  Precautions  in  reference  to 
barometers. — In  constructing  barometers, 
mercury  is  chosen  in  preference  to  any  other 
liquid.  For  being  the  densest  of  all  liquids 
it  stands  at  the  least  height  When  the 
mercurial  barometer  stands  at  thirty  inches, 
the  water  barometer  would  stand  at  about 
thirty-four  feet.  It  also  deserves  preference 
because  it  does  not  moisten  the  glass.  It 
is  necessary  that  the  mercury  be  pure  and 
free  from  oxide  ;  otherwise  it  adheres  to 
the  glass  and  tarnishes  it.  Moreover,  if 
it  is  impure  its  density  is  changed,  and 
the  height  of  the  barometer  is  too  great 
or  too  small.  Mercury  is  purified,  before 
being  used  for  barometers,  by  treatment  with 
dilute  nitric  acid,  and  by  distillation. 

The  space  at  the  top  of  the  tube  (figs.  96  and  99),  which  is  called 
the   Torricellian  vacuum,  must  be  quite  free  from  air  and  from 


Fig.  99- 


-125]      Variations  in  the  Height  of  the  Barometer.       1 1 1 

aqueous  vapour,  for  otherwise  either  would  depress  the  mercurial 
column.  Now,  glass  tubes  always  condense  aqueous  vapour  on 
their  surface.  Under  the  ordinary  pressure  of  the  atmosphere 
this  layer  of  moisture  adheres  to  the  glass  ;  but  in  a  vacuum 
where  there  is  no  pressure  it  escapes,  and  there  is  formed  a  mix- 
ture of  air  and  aqueous  vapour  which  depresses  the  mercurial 
column. 

The  air  and  moisture  ran  only  be  got  rid  of  by  boiling  the 
mercury  in  the  tube.  To  obtain  this  result,  a  small  quantity  of 
pure  mercury  is  placed  in  the  tube  and  boiled  for  some  time,  fig.  100. 


Fig.  too. 

It  is  then  allowed  to  cool,  and  a  further  quantity,  previously  warmed 
added,  which  is  boiled,  and  so  on,  until  the  tube  is  quite  full  ;  in 
this  manner  the  moisture  and  the  air  which  adhere  to  the  sides  of 
the  tube  pass  off  with  the  mercurial  vapour.  The  bulb  at  the  end 
is  placed  there  to  collect  the  mercury  which  may  distil  over.  It  is 
afterwards  removed. 

A  barometer  is  free  from  air  and  moisture  if,  when  it  is  inclined, 
the  mercury  strikes  with  a  sharp  metallic  sound  against  the  top  of 
the  tube.  If  there  is  air  or  moisture  in  it,  the  sound  is  deadened, 

125.  Variations  in  the  height  of  the  barometer. — When  the 
barometer  is  observed  for  several  days,  its  height  is  found  to  vary 
in  the  same  place,  not  only  from  one  day  to  another,  but  also  during 
the  same  day. 

The  extent  of  these  variations,  that  is,  the  difference  between  the 
greatest  and  the  least  height,  is  different  in  different  places.  It  in- 
creases from  the  equator  towards  the  poles.  The  greatest  variations 
are  observed  in  winter. 


112  On  Gases.  [125- 

The  mean  daily  height  is  the  height  obtained  by  dividing  the  sum 
of  24  successive  hourly  observations  by  24.  In  our  latitudes,  the 
barometric  height  at  noon  corresponds  to  the  mean  daily  height. 

The  mean  monthly  height  is  obtained  by  adding  together  the 
mean  daily  heights  for  a  month,  and  dividing  by  30. 

The  mean  yearly  height  is  similarly  obtained. 

Under  the  equator,  the  mean  annual  height  at  the  level  of  the  sea 
is  om"758,  or  29-14  inches.  It  increases  from  the  equator,  and 
between  the  latitudes  30°  and  40°,  it  attains  a  maximum  of  om763, 
or  30-04  inches.  In  lower  latitudes  it  decreases,  and  in  Paris  it 
does  not  exceed  om7568. 

The  general  mean  at  the  level  of  the  sea  is  om76i  or  29-96 
inches. 

The  mean  monthly  height  is  greater  in  winter  than  in  summer, 
in  consequence  of  the  cooler  atmosphere. 

Two  kinds  of  variations  are  observed  in  the  barometer :  ist,  the 
accidental  variations,  which  present  no  regularity  ;  they  depend  on 
the  seasons,  the  direction  of  the  winds,  and  the  geographical 
position,  and  are  common  in  our  climates  :  2nd,  the  daily  varia- 
tions, which  are  produced  periodically  at  certain  hours  of  the  day. 

At  the  equator,  and  between  the  tropics,  no  accidental  variations 
are  observed  ;  but  the  daily  variations  take  place  with  such  regu- 
larity that  a  barometer  may  serve  to  a  certain  extent  as  a  clock. 
The  barometer  sinks  from  midday  till  towards  four  o'clock  ;  it 
then  rises,  and  reaches  its  maximum  at  about  ten  o'clock  in  the 
evening.  It  then  again  sinks,  and  reaches  a  second  minimum 
towards  four  o'clock  in  the  morning,  and  a  second  maximum  at  ten 
o'clock. 

In  the  temperate  zones  there  are  also  daily  variations,  but  they 
are  detected  with  difficulty,  since  they  occur  in  conjunction  with  ac- 
cidental variations. 

The  hours  of  the  maxima  and  minima  appear  to  be  the  same  in 
all  climates,  whatever  be  the  latitude  ;  they  merely  vary  a  little  with 
the  seasons. 

126.  Causes  of  barometric  variations. —  It  is  observed  that 
the  course  of  the  barometer  is  generally  in  the  opposite  direction  to 
that  of  the  thermometer;  that  is.  that  when  the  temperature  rises 
the  barometer  falls,  and  vice  versa  ;  which  indicates  that  the 
barometric  variations  at  any  given  place  are  produced  by  the  ex- 
pansion or  contraction  of  the  air,  and  therefore  by  its  change  in 
density.  If  the  temperature  were  the  same  throughout  the  whole 
extent  of  the  atmosphere,  no  currents  would  be  produced,  and  at 


-127]  Barometric  Variations.  1 1 3 

the  same  height^  the  atmospheric  pressure  would  be  everywhere 
the  same.  But  when  any  portion  of  the  atmosphere  becomes 
warmer  than  the  neighbouring  parts,  its  specific  gravity  is  dimin- 
ished, and  it  rises  and  passes  away  through  the  upper  regions  of 
the  atmosphere ;  whence  it  follows  that  the  pressure  is  diminished, 
and  the  barometer  falls.  If  any  portion  of  the  atmosphere  retains 
its  temperature,  while  the  neighbouring  parts  become  cooler,  the 
same  effect  is  produced  ;  for  in  this  case,  too,  the  density  of  the 
first-mentioned  portion  is  less  than  that  of  the  others.  Hence,  also, 
it  usually  happens,  that  an  extraordinary  fall  of  the  barometer  at 
one  place  is  counterbalanced  by  an  extraordinary  rise  at  another 
place.  The  daily  variations  appear  to  result  from  the  expansions' 
and  contractions  which  are  periodically  produced  in  the  atmosphere 
by  the  heat  of  the  sun  during  the  rotation  of  the  earth. 

127.  Relation  of  barometric  variations  to  the  state  of  the 

V 

weather. — It  has  been  observed  that,  in  our  climate,  the  baro- 
meter in  fine  weather  is  generally  above  30  inches,  and  is  below 
this  point  when  there  is  rain,  snow,  wind,  or  storm,  and  also,  that 
for  any  given  number  of  days  on  which  the  barometer  stands  at  30 
inches,  there  are  as  many  fine  as  rainy  days.  From  this  coinci- 
dence between  the  height  of  the  barometer  and  the  state  of  the 
weather,  the  following  indications  have  been  marked  on  the 
barometer,  counting  by  thirds  of  an  inch  above  and  below  30 
inches  : 

Height  State  of  the  weather 

3 1    inches  .         .         .         .         .        .     Very  dry. 

3of      „      .         .         .         ,  .     Settled  weather. 

30^       „      .         .         .        .        .        .     Fine  weather. 

30        „ Variable. 

29!       „ Rain  or  wind. 

29^      „.,....     Much  rain. 

29        „  ...        .        .         .     Tempest. 

In  using  the  barometer  as  an  indicator  of  the  state  of  the  weather, 
we  must  not  forget  that  it  really  only  serves  to  measure  the  weight 
of.  the  atmosphere  and  that  it  only  rises  or  falls  as  this  weight  in- 
creases or  diminishes  ;  and  although  a  change  of  weather  fre- 
quently coincides  with  a  change  in  the  pressure,  they  are  not 
necessarily  connected.  This  coincidence  arises  from  meteorolo- 
gical conditions  peculiar  to  our  climate,  and  does  not  always  occur. 
That  a  fall  in  the  barometer  usually  precedes  rain  in  our  latitudes, 
is  caused  by  the  position  of  Europe.  The  south-west  winds,  which 

I 


114 


On  Gases. 


[127- 


are  hot,  and  consequently  light,  make  the  barometer  sink  ;  but  at 
the  same  time  as  they  become  charged  with  aqueous  vapour  in 
crossing  the  ocean,  they  bring  us  rain.  The  winds  of  the  north 
and  north-east,  on  the  contrary,  being  colder  and  denser,  make  the 


Fig,  101.  Fig.  102. 

barometer  rise  ;  and,  as  they  only  reach  us  after  having  passed  over 
vast  continents,  they  are  generally  dry. 

When  the  barometer  rises  or  sinks  slowly,  that  is,-  for  two  or 


Determination  of  the  Heights  of  Places.  115 

three  days,  towards  fine  weather  or  towards  rain,  it  has  been  found, 
from  a  great  number  of  observations,  that  the  indications  are  then 
extremely  probable.  Sudden  variations  in  either  direction  indicate 
bad  weather  or  wind. 

1 23.  Wheel  barometer. — The  wheel  barometer,  which  was  in- 
vented by  Hooke,  is  a  syphon  barometer,  and  is  especially  intended 
to  indicate  good  and  bad  weather  (fig.  101).  In  the  shorter  leg  of 
the  syphon  there  is  a  float  a,  which  rises  and  falls  with  the  mercury 
(fig.  102),  A  string  attached  to  this  float  passes  round  a  pulley, 
and  at  the  other  end  there  is  another  and  somewhat  lighter  weight. 
A  needle  fixed  to  the  pulley  moves  round  a  graduated  circle,  on 
which  is  marked  variable,  rain,  fiiu  weather,  etc.  When  the 
pressure  varies  the  float  sinks  or  rises,  and  moves  the  needle  round 
to  the  corresponding  points  on  the  scale. 

The  barometers  ordinarily  met  with  in  houses,  and  which  are 
called  weather  glasses,  are  of  this  kind.  They  are,  however,  of 
little  use,  for  two  reasons.  The  first  is,  that  they  are  neither  very 
delicate  nor  precise  in  their  indications.  The  second,  which  applies 
equally  to  all  barometers,  is,  that  those  commonly  in  use  in  this 
country  are  made  in  London,  and  the  indications,  if  they  are  of  any 
value,  are  only  so  for  a  place  at  the  same  level  and  of  the  same 
climatic  conditions  as  London.  Thus  a  barometer  standing  at  a 
certain  height  in  London  would  indicate  a  certain  state  of  weather, 
but  if  removed  to  Shooter's  Hill  it  would  stand  half  an  inch  lower, 
and  would  indicate  a  different  state  of  weather.  As  the  pressure 
differs  with  the  level  and  with  geographical  conditions,  it  is  neces- 
sary to  take  these  into  account  if  exact  data  are  wanted. 

129,  Determination  of  the  heights  of  places  by  the  baro- 
meter.— One  of  the  most  important  of  the  uses  of  the  barometer 
has  been  its  application  to  the  measurement  of  the  heights  of 
places  above  the  sea  level.  For,  if  we  suppose  the  atmosphere 
divided  into  horizontal  layers  of  equal  thickness,  a  hundred  for 
instance,  a  barometer  at  the  sea  level  would  support  the  weight  of 
a  hundred  of  these  layers  ;  and,  as  we  have  seen  (117),  would  be  at 
rest  when  its  height  was  thirty  inches.  If  it  were  raised  in  the  atmo- 
sphere to  the  height  of  ten  such  layers,  it  would  now  only  support 
the  weight  of  ninety  such  layers,  and  the  mercury  would  therefore 
necessarily  sink.  It  would  sink  still  further  if  it  were  raised  to  the 
twentieth  layer,  and  so  on  to  the  limit  of  the  atmosphere  if  that 
were  possible.  There  it  would  be  under  no  pressure,  and  the  level 
of  the  mercury  in  the  tube  and  in  the  cistern  would  be  the  same. 

I  2 


Ii6  On  Gases.  [129- 

As  the  mercury  sinks  in  proportion  as  we  rise  in  the  atmosphere, 
we  might,  from  the  amount  by  which  it  is  lower,  deduce  the  height 
above  the  sea  level.  If  air  had  everywhere  the  same  density  up 
to  the  extreme  limit  of  the  atmosphere,  the  calculation  would  be 
very  simple  ;  for  as  mercury  is  about  10,500  times  as  heavy  as  air, 
an  inch  of  the  barometer  would  correspond  to  a  column  of  air  about 
875  feet ;  hence,  in  ascending  a  mountain,  a  diminution  of  an  inch 
in  the  height  of  the  barometer  would  correspond  to  an  ascent  of 
about  875  feet.  But  the  density  of  the  air  decreases  as  we  ascend, 
for  the  layers  of  air  necessarily  support  a  less  weight  ;  hence,  the 
measurement  of  the  heights  by  the  barometer  is  not  so  simple  as 
we  have  supposed.  Very  complete  tables  have,  however,  been 
constructed,  by  which  the  difference  in  height  between  any  two 
places  may  be  readily  ascertained,  if  we  know  the  corresponding 
heights  of  the  barometer.  For  small  elevations  we  may  assume 
that  an  ascent  of  900  feet  produces  a  depression  of  an  inch  in  the 
height  of  the  barometer.  For  measuring  heights  by  the  barometer 
the  aneroid  (135)  is  extremely  convenient. 

130.  Height  of  the  atmosphere. — In  virtue  of  the  expansive 
force  of  the  air,  it  might  be  supposed  that  the  molecules  would 
expand  indefinitely  into  the  planetary  spaces.     But,  in  proportion 
as  the  air  expands,  its  expansive  force  decreases,  and  is  further 
weakened   by  the  low  temperature   of  the  upper   regions  of  the 
atmosphere,  so  that,  at  a  certain  height,  an  equilibrium  is  estab- 
lished between  the  expansive  force  which  separates  the  molecules, 
and  the  action  of  gravity  which  draws  them  towards  the  centre  of 
the  earth.     It  is  therefore  concluded  that  the  atmosphere  is  limited. 

From  the  weight  of  the  atmosphere,  and  its  decrease  in  density, 
and  from  the  observation  of  certain  phenomena  of  twilight,  its 
height  has  been  estimated  at  from  30  to  40  miles.  Above  that 
height  the  air  is  extremely  rarefied,  and  at  a  height  of  60  miles  it 
is  assumed  that  there  is  a  perfect  vacuum.  From  certain  observa- 
tions recently  made  in  the  tropical  zone,  and  particularly  at  Rio 
Janeiro,  on  the  twilight  arc,  M.  Liais  estimates  the  height  of  the 
atmosphere  at  between  198  and  212  miles,  considerably  higher, 
therefore,  than  what  has  hitherto  been  believed. 

ILLUSTRATIONS   OF  ATMOSPHERIC   PRESSURE. 

131.  The  pressure  of  the  atmosphere  is  transmitted  in  all 
directions.— The   atmosphere,  like  any  other  mass  of  fluid  (76), 


-132]       Pressure  supported  by  the  Hitman  Body.          1 1 7 

must  necessarily  transmit  its  pressure  in  all  directions,  upwards 
and  laterally  as  well  as  downwards.  We  have  already  seen  a 
striking  instance  of  this  in  the  Magdeburg  hemispheres  (116),  and 
the  following  experiment  furnishes  another  illustration  of  this 
point. 

A  tumbler  full  of  water  is  carefully  covered  with  a  sheet  of  paper, 
which  is  kept  in  position  by  one  hand,  while  with  the  other  the 
tumbler  is  inverted.  Removing  then  the  hand  which  held  the 
paper,  the  water  does  not  fall  out,  both  water  and  paper  being  kept 
in  position  by  the  upward  pressure  (fig.  103).  The  object  of  the 
paper  is  to  present  a  flat  surface  of  water,  for  otherwise  the  water 
would  divide  and  would  allow  air  to  enter,  and  then  the. experiment 
would  fail. 

The  use  of  the  wine-tester  also  depends  on  the  pressure  of  the 


Fig.  103.  Fig.  104. 

atmosphere.  It  consists  of  a  tin  tube  (fig.  104),  terminating  at  the 
bottom  in  a  small  cone,  the  end  of  which,  o,  is  open ;  at  the  top 
there  .is  a  small  aperture,  which  is  closed  by  the  thumb.  The  two 
ends  being  open,  the  tube  is  immersed  in  the  liquid  to  be  tested  ; 
closing  then  the  upper  end  .by  the  thumb,  as  shown  in  the  figure, 
the  tube  is  withdrawn,  and  remains  filled  in  consequence  of  the 
pressure  at  o.  But  if  the  thumb  be  withdrawn  the  pressure  is 
transmitted  both  upwards  and  downwards,  and  the  liquid  flows  out 
in  obedience  to  the  action  of  gravity. 

132.  Pressure  supported  by  tne  human  body. — The  surface 
of  the  body  of  a  man  of  middle  size  is  about  16  square  feet  ;  the 
pressure,  therefore,  which  a  man  supports  on  the  surface  of  his 
body  is  37,560  pounds,  or  upwards  of  16  tons.  Such  an  enormous 
pressure  might  seem  impossible  to  be  borne  ;  but  it  must  be  re- 
membered that  in  all  directions  there  are  equal  and  contrary  pres- 
sures which  counterbalance  one  another.  It  might  also  be  supposed 


On  Gases. 


[132- 


that  the  effect  of  this  force,  acting  in  all  directions,  would  be  to 
press  the  body  together  and  crush  it.  But  the  solid  parts  of  the 
skeleton  could  resist  a  far  greater  pressure  ;  and  as  to  the  liquids 
contained  in  the  organs  and  vessels,  from  what  has  been  said  about 
liquids  (75),  it  is  clear  that  they  are  virtually  incompressible.  The 
gases,  too,  are  compressed  by  the  weight 
of  the  atmosphere,  but  they  resist  it  in 
virtue  of  their  elasticity.  They  are,  in 
short,  like  a  bottle  full  of  air.  The  sides 
of  the  latter  are  pressed  in  by  the  weight 
of  the  atmosphere;  but  they  can  stand  this. 
however  thin  their  walls,  for  the  pressure 
of  the  gas  from  within  quite  counter- 
balances that  which  presses  on  the  out- 
side. 

The  following  experiment  (fig.  105) 
illustrates  the  effect  of  atmospheric 
pressure  on  the  human  body.  A  glass 
vessel  open  at  both  ends,  being  placed 
on  the  plate  of  the  air-pump,  the  upper 
Fig.  105.  end  of  the  cylinder  is  closed  by  the  hand 

and  a  vacuum  is  made.  The  hand  then  becomes  pressed  by  the 
weight  of  the  atmosphere,  and  can  only  be  taken  away  by  a  great 
effort.  And  as  the  elasticity  of  the  gas  contained  in  the  organs  is 
not  counterbalanced  by  the  weight  of  the  atmosphere,  the  palm  of 
the  hand  swells,  and  blood  tends  to  escape  from  the  pores. 

The  operation  of  cupping  in  medicine  is  an  application  of  the 
effect  of  removing  the  atmospheric  pressure  from  the  human  body. 
The  human  mouth  applied  upon  any  part,  in  the  action  of  sucking, 
is  a  kind  of  cupping  apparatus.  The  mouth  of  the  leech  is  such  an 
apparatus  with  one  lancet. 


CHAPTER   II. 

MEASUREMENT  OF  THE  ELASTIC   FORCE  OF  GASES. 

133.  Boyle's  law.— The  law  of  the  compressibility  of  gases  was 
discovered  by  Boyle,  and  subsequently,  though  independently,  by 
Mariotte.  In  consequence  it  is  in  England  commonly  called 
Boyle's  law,  and  on  the  Continent  Mariotte's  law.  It  is  as  follows  : 


-133] 


Boyle's  Law. 


119 


'  The  temperature   remaining  the  same,  the   volume   of  a  given 
quantity  of  gas  is  inversely  as  the  pressure  which  it  bears. 

This  law  is  verified  by  means  of  an  apparatus  called  Mariotte's 
tube  (fig.  1 06).     It  consists  of  a  long  glass  tube  fixed  to  a  vertical 


Fig.  106.  Fig.  107. 

support  :  it  is  open  at  the  top  ;  and  the  other  end,  which  is  bent 
into  a  short  vertical  leg,  is  closed.  On  the  shorter  leg  there  is  a 
scale,  which  indicates  equal  capacities ;  the  scale  against  the  long 
leg  gives  the  heights.  The  zero  in  both  scales  is  in  the  same  hori- 
zontal line. 

A  small  quantity  ot  mercury  is  poured  into  the  tube,  so  that  its 
level  in  both  branches  is  at  zero,  which  is  effected  without  much 


120 


On  Gases. 


[133- 


difficulty.  The  air  in  the  short  leg  is  thus  under  the  ordinary  atmo- 
spheric pressure.  If  mercury  is  then  poured  into  the  longer  tube 
the  volume  of  the  air  in  the  smaller  tube  is  gradually  reduced.  If 
this  be  continued  until  it  is  only  one-half,  that  is,  until  it  is  reduced 
from  10  to  5,  as  shown  in  figure  107,  and  if  the  height  of  the  mer- 
curial column ,  C  A,  be  no  w  measured, 
it  will  be  found  exactly  equal  to  the 
height  of  the  barometer  at  the  time 
of  the  .  experiment.  The  pressure 
of  the  column  CA  is  therefore  equal 
to  an  atmosphere,  which,  with  the 
atmospheric  pressure  acting  on  the 
surface  of  the  column  at  C,  makes 
two  atmospheres.  Accordingly, by 
doubling  the  pressure,  the  volume 
of  the  gas  has  been  diminished  to 
one-half. 

If  mercury  be  poured  into  the 
longer  branch  until  the  volume  of 
the  air  is  reduced  to  one-third  its 
original  volume,  it  will  be  found 
that  the  distance  between  the  level 
of  the  two  tubes  is  equal  to  two 
barometric  columns.  The  pressure 
is  now  three  atmospheres,  while  the 
volume  is  reduced  to  one-third. 
Dulong  and  Petit  have  verified  the 
law  for  air  up  to  27  atmospheres, 
by  means  of  an  apparatus  analogous 
to  that  which  has  been  described. 

The  law  also  holds  good  in  the 
case  of  pressures  of  less  than  one 
atmosphere.  To  demonstrate  this, 
Fig.  109.  mercury  is  poured  into  a  graduated 
tube,  until  it  is  about  two-thirds  full,  the  rest  being  air.  It  is  then 
inverted  in  a  deep  trough  containing  mercury  (fig.  108),  and  lowered 
until  the  levels  of  the  mercury  inside  and  outside  the  tube  are  the 
same,  and  the  volume  AB,  which  is  then  under  a  pressure  of  one 
atmosphere,  is  noted.  The  tube  is  then  raised,  as  represented  in 
fig.  109,  until  the  volume  of  the  air,  AC,  is  doubled.  The  height  of 
the  mercury  in  the  tube,  above  the  mercury  in  the  trough,  is  then 


Fig.  108. 


-134]  Manometers.  1 2 1 

found  to  be  exactly  half  the  height  of  the  barometer  at  the  time  of 
the  experiment.  Accordingly,  for  half  the  pressure  the  volume  has 
been  doubled. 

In  the  experiment  with  Mariotte's  tube,  as  the  quantity  of  air 
remains  the  same,  its  density  must  obviously  increase  as  its  volume 
diminishes,  and  vice  versa.  The  law  may  thus  be  enunciated  : 
'  For  the  same  temperature  the  density  of  a  gas  is  proportional  to' its 
pressure'  Hence,  as  water  is  773  times  as  heavy  as  air,  under  a 
pressure  of  773  atmospheres  air  would  be  as  dense  as  water. 

Until  within  the  last  few  years  Boyle's  law  was  supposed  to  be 
absolutely  true  for  all  gases  at  all  pressures  ;  but  several  physicists 
have  since  observed  that  the  gas  is  not  rigorously  exact,  especially 
in  the  case  of  those  gases  which  can  be  liquefied.  They  are  more 
compressed  than  is  required  by  the  law.  For  air,  Dulong  and 
Arago  investigated  the  pressure  up  to  27  atmospheres,  and  observed 
that  the  volume  of  air  always  diminished  a  little  more  than  is  re- 
quired by  Boyle's  law.  But,  as  these  differences  were  very  small, 
they  attributed  them  to  errors  of  observation,  and  concluded  that 
the  law  was  perfectly  exact,  at  any  rate  up  to  27  atmospheres. 

For  ordinary  pressures  Boyle's  law  may  be  assumed  to  be  exact 
for  all  gases. 

134.  Manometers. — Manometers  are  instruments  for  measuring 
the  elastic  force  of  gases  or  vapours.  In  all  manometers  the  unit 
chosen  is  the  pressure  of  one  atmosphere,  or  30  inches  of  mercury 
at  the  standard  temperature,  which,  as  we  have  seen,  is  nearly  1 5  Ibs. 
to  the  square  inch.  The  open  air  manometer  is  represented  in 
fig.  1 10  fixed  against  a  board  fastened  to  a  wall,  and  connected  with 
a  steam  boiler.  It  consists  of  a  glass  tube  about  20  feet  in  height 
open  at  the  top,  and  fixed  at  the  other  end  to  a  glass  bath  C,  con- 
taining mercury.  A  long  tube  connects  this  with  the  boiler. 

When  the  elastic  force  of  the  vapour  in  the  boiler  is  equal  to 
the  pressure  of  the  atmosphere,  it  will  counterpoise  the  weight  ot 
the  atmosphere  which  is  transmitted  through  the  tube,  and  the  level 
of  the  mercury  is  then  the  same  in  the  tube  and  in  the  bath.  At 
this  level  the  number  i  is  marked  on  the  board.  Then  since  a 
column  of  mercury  30  inches  in  height  represents  a  pressure  of  an 
atmosphere,  the  number  2  is  marked  at  this  height  above  i  ;  at  a 
height  of  30  inches  above  this  the  number  3  is  marked,  and  so  on, 
each  interval  of  30  inches  representing  an  atmosphere.  Thus,  for 
instance,  if  the  mercury  had  been  forced  up  to  3^,  as  represented  in 
the  drawing  that  would  indicate  that  the  tension  of  the  vapour  in 


22 


On  Gases. 


[134 


the  boiler  is  3^  atmospheres  ;  so  that,  on  each  square  inch  of  the 
internal  surface  of  the  boiler,  there  is  a  pressure  of  3^+15.  pounds, 
or  52 j  pounds. 

The  manometer  with  compressed  air  is  founded  on  Mariotte's 


law  :  it  consists  of  a  glass  tube  closed  at  the  top  (fig.  in),  and 
filled  with  dry  air.  It  is  firmly  cemented  in  a  small  bath  containing 
mercury.  By  a  tubulure,  this  bath  is  connected  with  the  closed 
vessel  containing  all  the  gas  or  vapour  whose  elastic  force  is  to  be 
measured. 


-135] 


Aneroid  Barometer. 


123 


In  the  graduation  of  this  manometer,  the  quantity  of  air  con- 
tained in  the  tube  is  such,  that  when  the  aperture  communicates 
freely  with  the  atmosphere,  the  level  of  the  mercury  is  the  same  in 
the  tube  and  in  the  bath.  Consequently,  at  this  level,  the  number 
i  is  marked  on  the  scale  to  which  the  tube  is  affixed.  As  the 
pressure  acting  through  the  tubulure  A  increases,  the  mercury  rises 
in  the  tube,  until  its  weight  added  to  the  elastic  force  of  the  com- 
pressed air,  is  equal  to  the  external  pressure.  It  would  consequently 
be  incorrect  to  mark  two  atmospheres  in  the  middle  of  the  tube  ; 


Fig.  112. 

for  since  the  volume  of  the  air  is  reduced  to  one-half,  its  elastic 
force  is  equal  to  two  atmospheres,  and,  together  with  the  weight  of 
the  mercury  raised  in  the  tube,  is  therefore  more  than  two  atmo- 
spheres. The  position  of  the  number  is  a  little  below  the  middle, 
at  such  a  height  that  the  elastic  force  of  the  compressed  air, 
together  with  the  weight  of  the  mercury  in  the  tube,  is  equal 
to  two  atmospheres.  The  exact  position  of  the  numbers  2,  3,  4, 
etc.,  on  the  manometer  scale  can  only  be  determined  by  calcula- 
tion. 

135.  Aneroid  barometer. — This   instrument  derives  its  name 


124  On  Gases.  [135- 

from  the  circumstance  that  no  liquid  is  used  in  its  construction  (a, 
without,  vnpbc,  moist).  Fig.  112  represents  one  of  the  forms  of  these 
instruments  constructed  by  Mr.  Casella  ;  it  consists  of  a  cylindrical 
metal  box,  exhausted  of  air,  the  top  of  which  is  made  of  thin  cor- 
rugated metal,  so  elastic  that  it  readily  yields  to  alterations  in  the 
pressure  of  the  atmosphere. 

When  the  pressure  increases,  the  top  is  pressed  inwards  ;  when 
on  the  contrary  it  decreases,  the  elasticity  of  the  lid,  aided  by  a 
spring,  tends  to  move  it  in  the  opposite  direction.  These  motions 
are  transmitted  by  delicate  multiplying  levers  to  an  index  which 
moves  on  a  scale.  The  instrument  is  graduated  empirically  by 
comparing  its  indications  under  different  pressures  with  those  of  an 
ordinary  mercurial  barometer. 

The  aneroid  has  the  advantage  of  being  portable,  and  can  be 
constructed  of  such  delicacy  as  to  indicate  the  difference  in  pressure 
between  the  height  of  an  ordinary  table  and  the  ground.  It  is 
hence  much  used  in  surveying  and  in  determining  heights  in 
mountain  ascents.  But  it  is  somewhat  liable  to  get  out  of  repair, 
especially  when  it  has  been  subjected  to  great  variations  of  pres- 
sure :  and  its  indications  must  from  time  to  time  be  compared  by 
means  of  a  standard  barometer. 


MIXTURE   AND   SOLUTION   OF   CASES. 

1 36.  Laws  of  the  mixture  of  gases. — We  have  seen  that  liquids, 
when  they  do  not  act  chemically  on  each  other,  tend  continually  to 
separate,  and  to  become  superposed  in  the  order  of  their  densities. 
This  is  not  the  case  with  gases  ;  being  under  a  continual  tendency 
to  expand,  when  they  mix,  their  mixture  is  found  to  be  subject  to 
the  following  laws  : 

I.  Whatever  their  densities,  gases  mix  in  equal  proportions  in 
all  parts  of  the  vzssel  in  which  they  are  contained. 

II.  The  elastic  force  of  the  mixture  is  equal  to  the  sum  of  the 
elastic  forces  of  the  constituents. 

The  first  law  was  shown  experimentally  by  Berthollet,  by  means 
of  an  apparatus  represented  in  fig.  113.  It  consisted  of  two  glass 
globes  provided  with  stopcocks,  which  could  be  screwed  one  on  the 
other.  The  upper  globe  was  filled  with  hydrogen,  and  the  lower 
one  with  carbonic  acid,  which  has  22  times  the  density  of  hydrogen. 
The  globes  having  been  fixed  together  were  placed  in  the  cellars  of 
the  Paris  Observatory,  and  the  stopcocks  then  opened,  the  globe 


-137] 


Mixture  of  Gases  and  Liquids. 


12=; 


containing  hydrogen   being   uppermost.       Bertholiet   found,    after 

some  time,  that  the  pressure  had  not  changed,  and  that,  in  spite 

of  the  difference   in  density,  the  two 

gases  had  becorne  uniformly  mixed  in 

the  two   globes.      Experiments  made 

in  the  same  manner  with  other  gases 

gave  the  same  results,  and  it  was  found 

that  the  diffusion  was  more  rapid  in 

proportion  as  the  difference  between 

the  densities  was  greater. 

In  accordance  with  this  law,  air 
being  a  mixture  of  nitrogen  and  oxy- 
gen, which  are  different  in  density,  its 
composition  should  be  the  same 
in  all  parts  of  the  atmosphere, 
which  in  fact  is  what  has  been  ob- 
served. 

Gaseous  mixtures  follow  Boyle's 
law,  like  simple  gases,  as  has  been 
proved  for  air  (133),  which  is  a  mix- 
ture of  nitrogen  and  oxygen. 

137.  Mixture  of  gases  and  liquids. 
many  liquids  possess  the  property  of  absorbing  gases.  Under  the 
same  conditions  of  pressure  and  temperature  a  liquid  does  not 
absorb  equal  quantities  of  different  gases.  At  the  ordinary  tem- 
perature and  pressure  water  dissolves  j2^~  its  volume  of  nitrogen, 
Y*5o  its  volume  of  oxygen,  its  own  volume  of  carbonic  acid,  and  430 
times  its  volume  of  ammoniacal  gas. 

The  general  laws  of  gas-absorption  are  the  following  : 

I .  For  the  same  gas,  the  same  liquid,  and  the  same  temperature ', 
the  weight  of  gas  absorbed  is  proportional  to  the  pressure.  This 
may  also  be  expressed  by  saying  that  at  all  pressures  the  volume 
dissolved  is  the  same  ;  or  that  the  density  of  the  gas  absorbed  is  in 
a  constant  relation  with  that  of  the  external  gas  which  is  not  ab- 
sorbed. 

Accordingly,  when  the  pressure  diminishes,  the  quantity  of  dis- 
solved gas  decreases.  If  a  solution  of  a  gas  be  placed  under  the 
air-pump  and  a  vacuum  created,  the  gas  obeys  its  expansive  force 
and  escapes  with  effervescence. 

The  manufacture  of  aerated  water  is  a  practical  application  of 
this  law.  By  means  of  force-pumps  an  excess  of  carbonic  acid  is 


Fig.  113. 
Absorption. — Water  and 


126  On  Gases.  [137- 

dissolved  in  the  water,  and  the  solution   is  then  preserved  in  care- 
fully closed  vessels. 

It  is  the  carbonic  acid  dissolved  in  beer,  in  champagne,  and  in 
all  effervescing  liquids,  which,  rapidly  escaping  when  the  bottles 
are  uncorked,  produces  the  well-known  report,  and  carries  with  it 
a  greater  or  less  quantity  of  the  liquid. 

I 1.  The  quantity  of  gas  absorbed  is  greater  when  the  temperature 
is  lower;  that  is  to  say,  when  the  elastic  force  of  the  gas  is  less. 

III.  The  quantity  of  gas  which  a  liquid  can  dissolve  is  indepen- 
dent of  the  nature  and  of  the  quantity  of  other  gases  which  it  may 
already  hold  in  solution. 


CHAPTER    III. 
APPARATUS   FOUNDED   ON   THE   PROPERTIES   OF  AIR. 

138.  Air-pump. — The  air-pump  is  an  instrument  by  which  a 
vacuum  can  be  produced  in  a  given  space,  or  rather  by  which  air 
can  be  greatly  rarefied,  for  an  absolute  vacuum  cannot  be  pro- 
duced by  its  means.  It  was  invented  by  Otto  von  Guericke  in 
1650,  a  few  years  after  the  invention  of  the  barometer. 

Fig.  114  gives  a  perspective  view  of  the  pump,  fig.  115  gives  a 
detailed  longitudinal  section,  and  fig.  116  gives  a  cross  section. 

The  pump  consists  of  two  stout  glass  barrels  in  which  twD  pis- 
tons, P  and  Q,  made  of  leather  well  soaked  with  oil,  move  up  and 
down,  and  close  the  barrels  air-tight.  The  pistons  are  fixed  to  two 
racks,  A  and  B,  working  with  a  pinion  (K,  fig.  1 16),  which  is  moved 
by  a  handle  MN,  so  that,  when  one  piston  rises,  the  other  de- 
scends. 

The  two  barrels  are  firmly  cemented  on  the  base,  H,  which  is  of 
brass  ;  on  this  plate  is  a  column,  I,  terminated  by  a  plate  G.  On 
this  plate  is  a  glass  bell  jar  which  is  called  the  receiver.  In  the 
interior  of  the  column  is  a  conduit,  which  is  prolonged  below  the 
base  to  between  the  two  barrels.  It  there  branches  in  the  shape  of 
a  T,  terminating  in  two  apertures,  a  and  b,  in  the  bottom  of  the 
cylinders.  These  apertures  are  conical,  and  are  closed  by  two 
small  conical  valves  ;  these  latter  are  fixed  to  metal  rods  which 
work  air-tight,  but  with  gentle  friction  in  the  pistons.  In  the  pistons 
is  a  cylindrical  cavity  communicating  with  the  lower  part  of  the 
pump  by  two  apertures,  s  and  t  (fig.  1 16).  These  apertures  are  closed 


-138] 


A  ir-pnmp. 


127 


by  small  clack  valves,  kept  in  position  by  springs  v/hich  surround 
the  rods  themselves.  The  four  valves,  a,  b,  s,  /,  it  may  be  remarked, 
open  upwards. 

These  details  being  known,  the  working  of  the  machine  is  readily 


Fig.  114. 

understood.  It  is  sufficient  to  consider  what  takes  place  in  a  single 
piston  (fig.  114).  The  piston  P  being  first  at  the  bottom  of  its 
stroke,  on  rising  it  raises  the  rod  which  traverses  it,  and  therewith 
the  valve  a,  which  remains  open  during  the  ascent.  The  valve,  /, 
which  is  in  the  piston,  remains  closed  by  the  action  of  the  spring 


128 


On  Gases. 


[138- 


and  the  pressure  of  the  atmosphere,  which  acts  in  the  barrel 
through  an  aperture,  r,  in  the  cover.  From  this  position  of  the 
two  valves,  it  will  be  seen  that,  as  the  piston  rises,  the  external 
pressure  of  the  atmosphere  cannot  act  in  the  bottom  of  the  barrel, 
but  the  air  of  the  receiver,  in  virtue  of  its  elasticity,  expands  and 
passes  by  the  conduit,  I  and  H,  into  the  barrel.  The  receiver  is 
still  full  of  air,  but  it  is  more  rarefied  ;  it  is  less  dense. 

When  the  piston  descends,  the  rod  which  bears  the  valve,  rt, 


Fig.  115. 

descending  with  it,  communication  between  the  receiver  and  the 
barrel  is  cut  ofY.  The  air  in  the  barrel  becomes  more  and  more 
compressed,  its  elastic  force  increases,  and  finally  overcomes  the 
atmospheric  pressure  ;  so  that  the  valve  t>  being  pressed  upwards 
by  the  elastic  force  of  the  air  in  the  interior  more  strongly  than  it 
is  pressed  downwards  by  the  atmosphere,  is  raised,  and  allows  the 
air  of  the  barrel  to  escape  into  the  upper  part  of  the  barrel,  and 
thence  into  the  atmosphere.  Thus  a  certain  quantity  of  air  has 


-139] 


A  ir-pump. 


129 


been  removed.  A  fresh  quantity  is  removed  at  a  second  stroke  of 
the  piston,  another  at  the  third,  and  so  on.  The  air  in  the  receiver 
is  thus  gradually  more  and  more  rarefied  ;  yet  all  the  air  cannot  be 
entirely  extracted,  for  it  ultimately  becomes  so  rarefied  both  in  the 
receiver  and  in  the  barrel,  that  when  the  piston  P  is  at  the  bottom 
of  its  stroke,  the  compressed  gas  below  the  piston  has  no  longer 
sufficient  force  to  overcome  the  resistance  of  the  atmosphere  and 
force  open  the  valve,  /.  The  limit  of  rarefaction  has  then  been 
attained,  and  it  is  useless  to  work 
the  pump  any  longer. 

What  has  been  said  in  refer- 
ence to  one  barrel  applies  also 
to  the  other.  The  machine 
works  with  one  ;  and  the  first 
air-pumps  had  but  one.  The 
advantage  of  having  two  is 
that  the  vacuum  is  more  rapidly 
produced.  The  use  of  double- 
action  air-pumps  was  first  intro- 
duced by  Hawksbee. 

139.  Measurement  of  the 
degree  of  rarefaction  in  the 
receiver.  —  Since  a  perfect 
vacuum  cannot  be  obtained  in 
the  receiver,  it  is  useful  to  have 
a  means  of  ascertaining  the  de- 
gree of  rarefaction  at  any  par- 
ticular time.  This  is  effected  by 
means  of  a  glass  cylinder,  E, 
connected  by  a  brass  cap  with 
the  conduit  in  the  column  I  Fig>  Il6- 

(fig.  114).  In  this  cylinder  is  placed  a  bent  glass  tube,  closed 
at  one  end  and  open  at  the  other.  This  is  called  the  air-pump 
gauge.  It  is  fixed  against  a  plate,  on  which  is  a  graduated  scale. 
The  closed  branch  being  at  first  full  of  mercury,  so  long  as  the 
air  in  the  receiver  P  and  in  the  cylinder  E  has  sufficient  tension, 
it  sustains  the  mercury  in  the  tube ;  the  height  of  which  is  from 
six  to  eight  inches.  But  as  the  pump  is  worked  the  air  becomes 
more  and  more  rarefied,  and  has  no  longer  the  elastic  force  suffi- 
cient for  retaining  the  column  of  mercury  in  the  closed  limb.  It 
accordingly  sinks  in  this  limb  and  rises  in  the  other.  The  greater 

K 


130 


On  Gases. 


[139- 


the  rarefaction,  the  smaller  the  difference  of  the  level  in  the  two  limbs. 
They  are,  however,  never  exactly  equal ;  that  would  correspond  to 
a  perfect  vacuum.  The  mercury  is  always  at  least  the  ^  tn  °f  an 
inch  higher  in  the  closed  branch.  This  is  expressed  by  saying  that 
a  vacuum  has  been  created  within  -5  th  of  an  inch. 

140.  Uses  of  tne  air-pump. — A  great  many  experiments  with 
the  air-pump  have  been  already  described.    Such  are  the  mercurial 

rain  (fig.  i),  the  fall  of  bodies  in 
vacuo  (fig.  37),  the  bladder  (fig.  87), 
the  bursting  of  a  bladder  (fig.  89),  the 
Magdeburg  hemispheres  (fig.  90),  and 
the  baroscope  (fig.  1 34). 

The  fountain  in  vacuo  (fig.  117)  is 
an  experiment  made  with  the  air- 
pump,  and  shows  well  the  elastic 
force  of  the  air.  It  is  a  flask  con- 
taining water  and  air  ;  the  neck  is 
closed  by  a  cork,  through  which  passes 
a  tube  dipping  in  the  liquid.  The  flask 
being  placed  under  the  receiver,  a  jet 
of  water  issues  from  the  top  of  the 
tube  as  soon  as  the  air  is  sufficiently 
rarefied.  This  is  due  to  the  elastic 
force  of  the  air  enclosed  in  the  flask. 

By  means  of  the  air-pump  it  may 
be  shown  that  air,  by  reason  of  the 
oxygen  it  contains,  is  necessary  for 
the  support  of  combustion  and  of  life. 
For  if  we  place  a  lighted  taper  under 
the  receiver  and  begin  to  exhaust  the 
air,  the  flame  becomes  weaker  as  rare- 
faction proceeds,  and  is  finally  extin- 
guished. Similarly  an  animal  faints 
and  dies,  if  a  vacuum  is  formed  in  a 
receiver  under  which  it  is  placed.  Mammalia  and  birds  soon 
die  in  vacuo.  Fishes  and  reptiles  support  the  loss  of  air  for  a  much 
longer  time.  Insects  can  live  several  days  in  vacuo. 

141.  Application    of    the    vacuum    to   the   preservation   of 
food. — An  important  application  has  been  made  of  the  vacuum  in 
preserving  food.     In  air,  under  the  influence  of  heat,  moisture,  and 
oxygen,  animal  and  vegetable  matters  rapidly  ferment  and  putrefy ; 


Fig.  117. 


-142] 


Condensing  Pump. 


but  if  the  oxygen  be  removed,  either  by  exhausting  or  by  other 
means,  they  may  be  kept  fresh  for  many  years. 

Appert  of  Paris  was  the  first  in  1 809  to  devise  a  means  of  pre- 
serving food  in  vacuo,  or  rather  in  a  space  deprived  of  oxygen, 
which  practically  amounts  to  the  same.  This  method  consists  in 
placing  the  substances  to  be  preserved  in  tin  vessels,  which  are 
closed  hermetically,  and  then  heated  in  boiling  water  for  some 
time  ;  under  the  influence  of  heat  the  small  quantity  of  oxygen  left 
•in  the  vessel  is  absorbed  by  the  substance  placed  there,  so  that 
only  nitrogen  is  present  in  the  free  state,  a  gas  which  cannot  induce 
fermentation.  Substances  properly  prepared  in  this  manner  may 
be  kept  for  years  without  alteration. 

Appert's  method  is  modified  in  England  in  the  following  manner. 
Instead  of  boiling  the  food  while 
Contained  in  the  closed  vessel, 
a  small  hole  is  left  in  the  lid, 
through  which  escape  the  air  and 
vapours  produced  during  ebulli- 
tion. When  it  is  supposed  that  all 
the  air  has  been  expelled,  a  drop 
of  melted  lead  is  allowed  to  fall 
on  the  small  hole  in  the  cover 
which  completely  closes  it.  This 
method  is  practised  on  the 
large  scale  in  preserving  food 
and  vegetables  for  the  use  of 
sailors,  and  also  in  preserving 
Australian  meat,  which  is  now 
consumed  in  large  quantities. 

142.  Condensing-  pump.— 
The  condensing  pump  is  an  ap- 
paratus for  compressing  air  or 
any  other  gas.  The  form  usually 
adopted  is  the  following  :  In  a 
cylinder,  A,  of  small  diameter 
(fig.  1 1 8),  there  is  a  solid  piston, 
the  rod  of  which  is  worked  by 
the  hand.  The  cylinder  is  pro- 
vided with  a  screw  which  fits  into  the  receiver,  K.  Fig.  119 
shows  the  arrangement  of  the  valves,  which  are  so  constructed 

K  2 


132  On  Gases.  [142- 

that  the  lateral  valve,  o,  opens  from  the  outside,  and  the  lower 
valve,  j,  from  the  inside. 

When  the  piston  descends,  the  valve  o  closes,  and  the  elastic 
force  of  the  compressed  air  opens  the  valve  s,  which  thus  allows 
the  compressed  air  to  pass  into  the  receiver.  When  the  piston 
ascends,  s  closes  and  o  opens,  and  permits  the  entrance  of  fresh 
air,  which  in  turn  becomes  compressed  by  the  descent  of  the  piston, 
and  so  on. 

This  apparatus  is  chiefly  used  for  charging  liquids  with  gases. 
For  this  purpose  the  stopcock  B  is  connected  with  a  reservoir  of 
the  gas,  by  means  of  the  tube  D.  The  pump  exhausts  this  gas, 
and  forces  it  into  the  vessel  K,  in  which  the  liquid  is  contained. 
This  can  be  drawn  off  by  the  stopcock  E.  The  artificial  aerated 
waters  are  made  by  means  of  analogous  apparatus. 

143.  Hero's  fountain. — Hero's  fountain  is  an  arrangement  by 
which  a  jet  may   be   obtained,  which  lasts  for   some  time.      It 
derives  its  name  from  its  inventor,  Hero,  who  lived  at  Alexandria. 
120  B.C.,  and  depends  on  the  elasticity  of  the  air.     It  consists  of 
a  brass  dish  (fig.   120),  and  of  two  glass  globes.     The  dish  com- 
municates with  the  lower  part  of  the  globe  by  a  long  tube  ;  and 
another  tube   connects   the  two   globes.      A   third    tube    passes 
through  the  dish  to  the  lower  part  of  the  upper  globe.     This  tube 
having  been  taken  out,  the  upper  globe  is  partially  filled  with  water, 
the  tube  is  then  replaced,  and  water  is  poured  into  the  dish.     The 
water  flows  through  the  long  tube  into  the  lower  globe,  and  expels 
the  air,  which  is  forced  into  the  upper  globe ;  the  air  thus  com- 
pressed, acts  upon  the  water,  and  makes  it  jet  out  as  represented 
in  the  figure.     If  it  were  not  for  the  resistance  of  the  atmosphere, 
and  friction,  the  liquid  would  rise  to  a  height  above  the  water  in 
the  dish  equal  to  the  difference  of  the  level  in  the  two  globes. 

144.  Intermittent   fountain. — The  intermittent  fountain  de- 
pends  partly  on   the   elastic  force  of  the  air  and  partly  on  the 
atmospheric  pressure.      It  consists  of  a  stoppered  glass  globe  a 
(fig.  121),  provided  with  two  or  three  capillary  tubulures.     A  glass 
tube,  d,  open  at  both  ends,  reaches  at  one  end  to  the  upper  part 
of  the  globe,  a  ;  the  other  end  is  fitted  in  a  support,  c,  placed 
in  the  middle  of  the  dish,  m,  which   supports   the  whole  appa- 
ratus.     The   support,    c,    is   perforated  with   small  holes,  which 
allow  air  to  pass  into  the  tube  just  above  a  little  aperture  in  the 
dish,  m. 

The  water,  with  which  the  globe,  a,  is  nearly  two-thirds  filled, 


-144] 


Intermittent  Fountain. 


133 


runs  out  by  the  tubes,  as  shown  in  the  figure  ;  the  internal  pres- 
sure being  equal  to  the  atmospheric  pressure,  together  with  the 
weight  of  the  column  of  water,  while  the  external  pressure  at  that 
point  is  only  that  of  the  atmosphere.  These  conditions  prevail  so 
long  as  the  lower  end  of  the  glass  tube  is  open,  that  is,  so  long  as 


Fig.  120. 

air  can  enter  and  keep  the  air  in  a  at  the  same  density  as  the  ex- 
ternal air  ;  but  the  apparatus  is  arranged  so  that  the  orifice  in  the 
dish  does  not  allow  so  much  water  to  flow  out  as  it  receives  from 
the  upper  tubes,  in  consequence  of  which  the  level  gradually  rises  in 


134 


On  Gases. 


[144- 


the  dish,  and  then  closes  the  lower  end  of  the  glass  tube.  As  the 
external  air  cannot  now  enter  the  globe,  a,  the  air  becomes  rarefied 
in  proportion  as  the  flow  continues,  until  the  pressure  of  the  column 
of  water,  together  with  the  elastic  force  of  the  air  contained  in 
the  globe,  is  equal  to  this  external  pressure  ;  the  flow  consequently 


Fig.  121. 

stops.  But  as  water  continues  to  flow  out  of  the  dish  at  m,  the 
tube  opens  again,  air  enters,  and  the  flow  recommences,  and  so  on, 
as  long  as  there  is  water  in  the  globe  a. 

145.  Syphon  inkstand. — This  instrument,  the  object  of  which 
is  to  protect  ink  from  too  rapid  evaporation  is  an  interesting  illus- 
tration of  the  pressure  of  the  atmosphere,  and  of  the  elasticity  of 


-146] 


Suction  Pump. 


135 


Fig.  122. 


air.  It  consists  of  a  glass  vessel  of  the  shape  of  a  truncated  pyra- 
mid (fig.  122),  closed  everywhere  except  at  the  bottom,  where  there 
is  a  tubulure,  which  is  always  open. 
The  inkstand  is  partially  full  of  ink, 
while  there  is  air  at  the  top.  The 
level  of  the  ink  inside  being  higher 
than  in  the  tubulure,  the  elastic  force 
of  the  air  inside  is  a  little  less  than 
the  pressure  of  the  atmosphere  on 
the  ink  in  the  tubulure.  As  the  ink 
there  is  used,  its  level  sinks,  and  is 
finally  lower  than  the  point  o.  At 
this  moment  a  bubble  of  air  passes 
into  the  interior,  and  the  elastic  force 
being  thereby  increased,  the  level  of 
the  ink  descends  in  the  inside  and 
rises  in  the  tubulure.  This  goes  on 
until  the  internal  level  is  at  the  point 
o.  More  ink  must  then  be  added,  which  is  effected  by  pouring 
it  into  the  tubulure,  care  being  taken  to  incline  the  inkstand  in  the 
opposite  direction.  The  fountains  in  birdcages  are  on  a  similar 
principle. 

PUMPS. 

146.  Suction-pump. — Pumps  are  machines  for  raising  liquids. 
Their  invention,  which  is  of  great  antiquity,  is  attributed  to 
Ctesibius.  a  celebrated  mechanician,  who  flourished  at  Alexandria, 
130  B.C.  There  are  many  modifications  of  pumps,  but  they  may 
all  be  referred  to  three  types,  the  suction  or  lift-pump,  the  forcing 
pump,  and  the  suction  arid  forcing  pump. 

The  suction  or  lifting  pump,  represented  in  fig.  123.  consists  of 
a  cast-iron  cylinder  called  the  barrel,  at  the  bottom  of  which  is  a 
pipe  of  a  smaller  diameter,  which  dips  in  the  well.  At  the  top  of 
this  pipe  is  a  clack  valve,  which  is  represented  in  the  drawing  as 
being  open.  It  moves  easily  up  and  down,  and  it  establishes  a 
communication  between  the  cylinder  and  the  body  of  the  pump 
when  it  is  open,  and  breaks  it  when  closed.  The  piston  in  the 
barrel  consists  of  a  thick  disc  of  metal  or  of  leather,  coated  with 
tow  or  with  leather.  The  piston  is  perforated  by  a  small  hole, 
which  is  closed  by  a  valve ;  the  valve  is  like  that  in  the  barrel, 
and,  like  it,  opens  upwards.  The  pistoh  is  worked  by  means  of 


136  On  Gases.  [146- 

a  long  lever,  which  is  the  handle.  This  is  joined  at  one  end  to  a 
forked'  rod,  a,  which  is  connected  with  the  piston  rod,  b.  As  it  is 
important  that  the  piston  move  in  a  straight  line,  it  is  guided  by 
passing  through  a  hole  in  a  fixed  piece,  o. 


Fig.  123. 

The  manner  in  which  the  water  is  raised  will  be  understood 
from  an  inspection  of  figs.  124,  125,  and  126,  which  represent  the 
piston  and  the  valves  in  three  different  positions.  When  the 
pump  has  not  been  worked,  the  barrel  and  the  pipe  are  full  of  air 
under  the  ordinary  atmospheric  pressure,  which  counterbalances 
the  external  atmospheric  pressure  on  the  well.  Hence  it  follows 


-146] 


Sziction  Pump. 


137 


that  the  level  of  the  water  inside  and  outside  is  the  same.  When 
the  piston  rises  (fig.  124),  as  it  is  pressed  down  by  its  own  weight 
and  by  that  of  the  atmosphere,  a  vacuum  is  created  below  the 
piston ;  but,  in  virtue  of  its  elastic  force,  the  air  which  fills  the 
pipe  B  quickly  opens  the  valve,  <z,  and  passes  into  the  barrel.  The 
air  in  the  pipe  B  losing  then  in  elastic  force  what  it  gains  in  volume 
(133),  its  pressure  is- no  longer  equal  to  the  external  pressure  on  the 


Fig.  124.  Fig.  125.  Fig.  126. 

water  in  the  well.  Hence  water  rises  in  the  pipe,  as  represented 
in  the  diagram.  If  now  the  piston  sinks  (fig.  125),  the  valve  a 
closes ;  and  as  the  air  thus  enclosed  in  the  barrel  becomes  more 
and  more  compressed,  a  moment  arrives  when  its  elastic  force 
exceeding  the  pressure  of  the  atmosphere,  the  valve  c  is  raised, 
and  air  escapes  into  the  top  of  the  barrel,  and  thence  into  the 
atmosphere.  With  a  second  ascending  stroke  of  the  piston,  the 
same  phenomena  are  reproduced  ;  that  is,  c  falls  and  the  valve  a 
opens,  the  water  being  thus  raised  in  the  pipe,  ultimately  passes 
beyond  the  valve  #,  and  completely  fills  the  barrel.  From  this 
time,  when  the  piston  re-descends,  and  the  valve  a  closes,  the 


138 


On  Gases. 


[146- 


pressure  exerted  on  the  water  raises  the  valve  c,  and  the  water 
passes  above  the  piston  (fig.  125).  Onee  this  effect  is  produced, 
when  the  piston  ascends,  the  valve  c  closes,  and  the  water  which 
has  passed  above  the  piston  being  raised  with  it,  ultimately  flows 
out  by  a  lateral  tubulure  in  the  barrel  (fig.  126). 

Since  it  is  the  atmospheric  pressure  which  raises  the  water  in 
the  pipe,  the  height  of  the  valve  a,  above  the  'level  in  the  vessel, 
cannot  exceed  a  certain  limit.  A  column  of  water,  34  feet  in 
height,  balances,  as  we  have  seen,  the  pressure  of  the  atmosphere 
(118).  Hence  if  the  pipe  had  a  greater  length  than  this,  when 
once  water  had  reached  this  height,  the  column  of  water  in  the 
pipe  would  balance  the  pressure  of  the  atmosphere  on  the  water 
of  the  well,  and  it  could  not  be  raised  any  higher.  This,  therefore, 
would  be  the  extreme  theoretical  limit  which  the  pipe  could  have  ; 
but  in  practice  the  height  of  the  tube  A  does  not  exceed  26  to  28 
feet ;  for,  although  the  atmospheric  pressure  can  support  a  higher 

column,  the  vacuum  produced  in  the 
barrel  is  never  perfect,  owing  to  the 
fact  that  the  piston  does  not  fit 
exactly  on  the  bottom  of  the  barrel. 
But  when  the  water  has  passed  the 
piston,  it  is  the  lifting  force  of 
the  latter  which  raises  it,  and  the 
height  to  which  it  can  be  brought 
depends  on  the  force  which  works 
the  piston. 

147.  Force-pump.  —  In  these 
water  is  not  raised  by  the  pressure 
of  the  atmosphere,  but  by  the 
pressure  of  the  piston  on  the  water 
during  its  descent.  For  this  pur- 
pose the  piston  is  solid,  that  is, 
has  no  valve,  and  there  is  no  lifting 
pipe,  the  barrel  being  immersed 
in  the  liquid  to  be  raised  (figs.  127 
and  128).  There  are  two  valves  in 
the  barrel  ;  one  a,  in  the  bottom, 

opens  upwards  ;  the  other,  £,  is  placed  in  the  orifice  of  a  long  tube 
in  the  side  of  the  pump. 

When  the  piston  rises  (fig.  1 27),  a  vacuum  being  produced  below 
it,  the  atmospheric  pressure  acts  on  c,  and  closes  it  ;  while  the  water 


-148] 


Fire-engines. 


in  which  the  pump  is  immersed  being  forced  by  its  own  weight  and 
that  of  the  atmosphere,  raises  the  valve  a,  and  passes  into  the 
barrel  which  it  fills  completely. 
The  motion  of  the  pistons  is  just 
reversed  when  the  piston  descends 
(fig.  1 28).  By  its  own  weight  and 
by  the  pressure  upon  it,  the  valve 
a  closes,  while  the  valve  c  opens 
and  gives  exit  to  the  water  in 
the  barrel,  which  then  rises  to  a 
height  depending  on  the  pressure 
exerted  by  the  piston.  If  this 
amounts  to  a  pressure  of  one  at- 
mosphere, water  rises  34  feet  in 
the  pipe  H  (118)  ;  if  it  is  two 
atmospheres  water  rises  to  68  feet, 
and  so  on  ;  that  is,  always  to  a 
height  of  34  feet  for  a  pressure 
of  one  atmosphere.  The  height, 
therefore,  to  which  water  can  be 
raised  in  these  pumps  is  not  limited 
as  it  is  in  the  suction-pump. 

From  what  has  been  said,  it 
will  be  seen  that  water  only  rises  in 
the  pipe  H  when  the  piston  descends 
mittent  flow  at  the  end  of  the  pipe, 
tained  by  arranging  two  pumps,  both  forcing  water  into  the  same 
pipe,  and  in  such  a  manner  that,  when  one  piston  rises,  the  other 
sinks.  It  is  by  means  of  such  an  arrangement  of  two  pumps,  that 
air  is  raised  to  the  wicks  in  Career's  lamp.  At  the  base  of  these 
lamps,  and  immersed  in  the  oil  itself,  are  two  small  pumps  worked 
by  a  clock-work  motion,  which  is  wound  up  like  a  clock.  Such  a 
system  is  also  applied  in  fire-engines. 

148.  Fire-engines. — In  a  fire-engine  water  has  to  be  forced  to 
a  great  height  in  a  continuous  stream.  Fig.  129  represents  a  section 
of  such  a  pump.  To  the  handles  PQ  are  fixed,  by  means  of  a  joint, 
two  rods  which  work  the  pistons  m  and  n  in  two  brass  barrels. 
These  pumps  are  placed  in  a  trough,  MN,  of  the  same  metal,  which 
is  called  the  tank,  and  which  is  fed  with  water  while  the  pump  is  at 
work.  Between  these  two  is  an  air-chamber  R,  with  a  lateral 
aperture  Z,  to  which  can  be  attached  a  long  leather  tube.  This 


Fig.  128. 

there  is,  therefore,  an  inter- 
A  more  regular  flow  is  ob- 


140 


On  Gases. 


[148- 


tube  is  provided  at  the  end  with  a  long  conical  copper  tube,  and 
which  has  an  aperture  only  about  three-fifths  of  an  inch  diameter. 

The  use  of  the  air-chamber  is  as  follows  :  Although  the  pistons 
work  alternately  there  would  necessarily  be  some  intermittence  in 
the  jet  when  they  are  at  the  top  or  at  the  bottom  of  their  course. 
But  the  water,  instead  ot  being  forced  by  the  pumps  directly  into 
the  ascending  pipe,  first  passes  into  the  reservoir  R,  as  shown  in 
fig.  129.  Owing  to  the  resistance  in  the  tube  and  on  the  jet,  it 


Fig.  129. 

flows  out  of  the  reservoir  more  slowly  than  it  enters.  Its  level  rises 
in  the  reservoir,  and  as  the  air  is  thereby  reduced  in  volume,  its 
pressure  increases,  so  that  the  compressed  air,  reacting  on  the  water 
when  the  pistons  stop,  forces  out  the  water  and  thus  keeps  up  the 
continuity  of  the  jet.  A  good  fire-engine  worked  by  eight  men  will 
raise  water  to  a  height  of  100  feet. 

149.  The  syphon. — The  syphon  is  a  bent  tube  open  at  both  ends 
and  with  unequal  legs  (fig.  130).  It  is  used  in  transferring  liquids, 
especially  in  cases  in  which  they  are  to  be  removed  without  dis- 
turbing any  sediment  they  contain.  It  is  worked  in  the  following 
manner  :  The  syphon  is  filled  with  some  liquid,  and  the  two  ends 
being  closed,  the  shorter  leg  is  dipped  in  the  liquid  as  represented 


-149] 


The  Syphon. 


141 


in  fig.  130 ;  or  the  shorter  leg  having  been  dipped  in  the  liquid,  the 
air  is  exhausted  by  applying  the  mouth  at  b.  A  vacuum  is  thus 
produced,  the  liquid  in  d  rises  and  fills  the  tube  in  consequence  of 
the  atmospheric  pressure.  It  will  then  run  out  through  the  syphon 
as  long  as  the  shorter  end  dips  in  the  liquid. 

A  syphon  of  the  form  represented  in  fig.  131  is  used  where  the 
presence  of  the  liquid  in  the  mouth  would  be  objectionable.  A 
tube,  rt,  is  attached  to  the  longer  branch,  and  it  is  filled  by  closing 
the  end  of  the  longer  limb?  and  sucking  at  the  end  of  a. 


Fig.  130. 


Fig.  131. 


To  explain  this  flow  ol  water  from  the  syphon,  let  us  suppose  it 
filled  and  the  short  leg  immersed  in  the  liquid.  The  pressure  then 
acting  on  d,  and  tending  to  raise  the  liquid  in  the  tube,  is  the  at- 
mospheric pressure  minus  the  height  of-  the  column  of  liquid,  cd. 
In  like  manner,  the  pressure  on  the  end  of  the  tube  b  is  the  weight 
of  the  atmosphere  less  the  pressure  of  the  column  of  liquid,  ab. 
But  as  this  latter  column  is  longer  than  cd,  the  force  acting  at  b  is 
less  than  the  force  acting  at  d,  and  consequently  a  flow  takes  place 
proportional  to  the  difference  between  these  two  forces.  The  flow 
will  therefore  be  more  rapid  in  proportion  as  the  difference  of  level 
between  the  aperture  b  and  the  surface  of  the  liquid  in  d  is 
greater. 


1 42 


On  Gases. 


[150 


150.  Intermittent  springs.  Tantalus'  cup. — In  nature,  springs 
are  met  with  whose  flow  is  spontaneously  interrupted,  and  which 
begins  again  after  a  longer  or  shorter  interval.  This  phenomenon 


Fig.  132. 

depends  on  the  action  of  the  syphon,  and  is  readily  understood  by 
reference  to  fig.  132,  which  represents  a  subterranean  reservoir  fed 
by  a  series  of  fissures  in  the  earth  ;  the  channel  by  which  the  water 
flows  out  is  on  the  left  of  the  figure,  and  on  coming  to  the  surface 
it  forms  a  spring.  In  the  figure  the  reservoir  is  represented  as  just 
being  filled,  and  when  the  water  rises  to  the  height  of  the  bend  the 
syphon  begins  to  act.  If  the  fissures 
by  which  water  is  supplied  furnish 
a  smaller  quantity  than  that  which 
flows  out,  the  reservoir  together  with 
the  channel  is  gradually  emptied,  and 
the  flow  then  ceases.  The  reservoir 
gradually  fills  again,  but  the  water 
cannot  flow  out  until  it  has  risen  to 
the  height  represented  by  the  dotted 
line,  and  the  syphon  has  begun  to 
Fig.  133.  work  again. 

The  action  of  Tantalus'  cup  which 

is  represented  in  fig.  1 33  will  be  at  once  understood ;  and  the  same 
principle  is  applied  in  an  arrangement  frequently  used  by  photo- 
graphers for  washing  prints. 


-151]        Archimedes  Principle  applied  to  Gases.  143 


CHAPTER   IV. 

PRESSURE  ON  BODIES  IN  AIR.   BALLOONS. 

151.  Archimedes'  principle  applied  to  gases. — The  pressure 
exerted  by  gases  on  bodies  immersed  in  them,  is  transmitted  equally 
in  all  directions,  as  has  been  shown  by 
the  experiment  with  the  Magdeburg 
hemispheres.  It  therefore  follows,  that 
all  which  has  been  said  about  the  equi- 
librium of  bodies  in  liquids,  applies  to 
bodies  in  air;  they  lose  a  part  of 
their  weight  equal  to  that  of  the  air 
which  they  displace. 

This  loss  of  weight  in  air  is  demon- 
strated by  means  of  the  baroscope, 
which  consists  of  a  scalebeam,  at  one 
of  whose  ends  a  small  leaden  weight 
is  supported,  and  at  the  other  there  is 
a  hollow  copper  sphere  (fig.  132). 
They  are  so  constructed  that  in  air 
they  exactly  balance  one  another,  but 
when  they  are  placed  under  the  re- 
ceiver of  the  air-pump  and  a  vacuum  is 
produced,  the  sphere  sinks;  thereby 

showing  that  in  reality  it  is  heavier  than  the  small  leaden  weight. 
Before  the  air  is  exhausted  each  body  is  buoyed  up  by  the  weight 
of  the  air  which  it  displaces.  But  as  the  sphere  is  much  the  larger 
of  the  two,  its  weight  undergoes  most  apparent  diminution  ;  and 
thus,  though  in  reality  the  heavier  body,  it  is  balanced  by  the  small 
leaden  weight.  It  may  be  proved  by  means  of  the  same  apparatus 
that  this  loss  is  equal  to  the  weight  of  the  displaced  air,  and  we 
may  thus  generalise  Archimedes'  principle  and  say,  that  any  body 
plunged  in  any  fluid,  whether  it  be  a  liquid  or  a  gas,  loses  part  of 
its  weight  equal  to  the  weight  of  the  displaced  fluid.  Hence  bodies 
weighed  in  air  usually  indicate  too  small  a  weight.  To  have  an 
exact  weight  the  volume  of  the  weights  and  of  the  displaced  fluid 
should  be  exactly  the  same,  which  is  seldom  the  case.  The  true 
weight  of  bodies  is  obtained  by  weighing  them  in  a  vacuum. 


• 


144  OH  Gases.  [151- 

The  principle  of  Archimedes  being  thus  true  for  bodies  in  air,  all 
that  has  been  said  about  bodies  immersed  in  liquids  applies  to 
them,  that  is,  that  when  a  body  is  heavier  than  air  it  will  sink, 
owing  to  the  excess  of  its  weight  over  the  buoyancy.  If  it  is  as  heavy 
as  air,  its  weight  will  exactly  counterbalance  the  buoyancy,  and  the 
body  will  float  in  the  atmosphere.  If  the  body  is  lighter  than  air, 
the  buoyancy  of  the  air  will  prevail,  and  the  body  will  rise  in  the 
atmosphere  until  it  reaches  a  layer  of  the  same  density  as  its  own. 
The  force  of  the  ascent  is  equal  to  the  excess  of  the  buoyancy  over 
the  weight  of  the  body.  This  is  the  reason  why  smoke,  vapours, 
clouds,  and  air  balloons  rise  in  the  air. 

152.  Air  balloons.— Air  balloons  are  hollow  spheres  made  of 
some  light  impermeable  material,  which,  when  filled  with  heated 
air,  with  hydrogen  gas,  or  with  coal  gas,  rise  in  the  air  in  virtue  of 
their  relative  lightness. 

They  were  invented  by  the  brothers  Montgolfier,  of  Annonay, 
and  the  first  experiment  was  made  at  that  place  in  June  1783. 
Their  balloon  was  a  sphere  of  40  yards  in  circumference,  and 
weighed  500  pounds.  At  the  lower  part  there  was  an  aperture,  and 
a  sort  of  boat  was  suspended,  in  which  was  burnt  paper  and  straw. 
The  heated  air  thus  produced  gradually  inflated  the  balloon,  and 
when  it  was  full  of  expanded  air,  which  was  thus  lighter  than  the 
external  air,  the  weight  of  the  balloon  and  its  hot  air  being  less  than 
that  of  the  air  which  it  displaced,  it  soon  rose  to  a  height  of  more 
than  2,000  yards,  to  the  great  astonishment  of  the  assembled 
spectators.  It  rapidly  descended,  however,  for  the  hot  air  it  con- 
tained soon  became  cooled  in  the  higher  regions  of  the  atmosphere. 

The  experiment  at  Annonay  excited  great  interest  all  over  France, 
and  pending  the  repetition  on  a  larger  scale  at  the  expense  of  the 
government,  Charles,  a  professor  of  physics,  constructed  a  smaller 
balloon,  about  1 3  feet  in  diameter,  which  was  filled  with  hydrogen 
instead  of  heated  air.  The  use  of  hydrogen  is  very  advantageous, 
for  as  it  is  almost  14  times  less  dense  than  air,  its  ascensional  force 
is  far  greater  than  that  of  hot  air,  and  it  is  also  less  dangerous, 
for  in  heating  the  air  there  is  a  great  risk  of  setting  fire  to  the 
balloon.  Charles  made  an  ascent  in  1783  in  a  balloon  inflated  by 
hydrogen. 

Since  then,  the  art  of  ballooning  has  been  greatly  extended,  and 
many  ascents  have  been  made.  That  which  Gay-Lussac  made  in 
1804  was  the  most  remarkable  for  the  facts  with  which  it  has  en- 
riched science,  and  for  the  height  which  he  attained — 23,000  feet 


-153]  Balloons.  145 

above  the  sea  level.  At  this  height  the  barometer  stood  at  12-6 
inches,  and  the  thermometer,  which  was  31°  C.  on  the  ground  was 
9  degrees  below  zero. 

In  these  high  regions,  the  dryness  was  such  on  the  day  of  Gay- 
Lussac's  ascent,  that  hygrometric  substances,  such  as  paper,  parch- 
ment, &c..  became  dried  and  crumpled  as  if  they  had  been  placed 
near  the  fire.  The  respiration  and  circulation  of  the  blood  were 
accelerated  in  consequence  of  the  great  rarefaction  of  the  air.  Gay- 
Lussac's  pulse  made  120  pulsations  in  a  minute,  instead  of  66,  the 
normal  number.  At  this  great  height  the  sky  had  a  very  dark 
blue  tint,  and  an  absolute  silence  prevailed. 

One  of  the  most  remarkable  recent  ascents  was  made  by  Mr. 
Glaisher  and  Mr.  Coxwell,  in  a  large  balloon  belonging  to  the 
latter.  This  was  filled  with  90,000  cubic  feet  of  coal  gas  (sp.  gr. 
0'37  to  0-33)  :  the  weight  of  the  load  was  600  pounds.  The  ascent 
took  place  at  I  P.M.  on  September  5,  1861  ;  at  i°  28'  they  had 
reached  a  height  of  15,750  feet,  and  in  eleven  minutes  afterwards  a 
height  of  21,000  feet,  the  temperature  being  — 10-4°;  at  i°  50'  they 
were  at  26,200  feet  with  the  thermometer  at-  15-2°.  At  i°  52'  the 
height  attained  was  39,000  feet,  and  the  temperature  -  16-0  C.  At 
this  height  the  rarefaction  of  the  air  was  so  great  and  the  cold  so 
intense  that  Mr.  Glaisher  fainted,  and  could  no  longer  observe. 
According  to  an  approximate  estimation  the  lowest  barometric 
height  they  attained  was  7  inches,  which  would  correspond  to  a 
height  of  36,000  to  37,000  feet. 

153.  Construction  and  management  of  balloons. — A  balloon 
(fig.  135)  is  made  of  long  bands  of  silk  sewed  together  and  covered 
with  caoutchouc  varnish,  which  renders  it  air-tight.  At  the  top 
there  is  a  safety-valve  closed  by  a  spring,  which  the  aeronaut  can 
open  at  pleasure  by  means  of  a  cord.  A  light  wicker-work  boat 
is  suspended  by  means  of  cords  to  a  net-work,  which  entirely  covers 
the  balloon. 

A  balloon  of  the  ordinary  dimensions,  which  can  carry  three 
persons,  is  about  16  yards  high,  12  yards  in  diameter,  and  its 
volume  when  it  is  quite  full  is  about  680  cubic  yards.  The  balloon 
itself  weighs  200  pounds  ;  the  accessories,  such  as  rope  and  boat, 
loo  pounds. 

The  balloon  is  filled  either  with  hydrogen  or  with  coal  gas.  Al- 
though the  latter  is  heavier  than  the  former,  it  is  generally  pre- 
ferred, because  it  is  cheaper  and  more  easily  obtained.  It  is  passed 
.into  the  balloon  from  the  gas  reservoir  by  means  of  a  flexible  pipe 

L 


Ou  Gases, 


[153- 


(hg.  135).  It  is  important  not  to  fill  the  balloon  quite  full,  for  the 
atmospheric  pressure  diminishes  as  it  rises,  and  the  gas  inside 
expanding  in  consequence  of  its  elastic  force,  tends  to  burst  it. 
It  is  sufficient  for  the  ascent  if  the  weight  of  the  displaced  air 
exceeds  that  of  the  balloon  by  8  or  10  pounds.  The  buoyancy 


Fig.  135- 

due  to  this  excess  of  weight  is  constant  so  long  as  the  balloon  is  not 
quite  distended  by  the  dilatation  of  the  air  in  the  interior.  If  the 
atmospheric  pressure,  for  example,  has  diminished  to  one-half,  the 
gas  in  the  balloon,  according  to  Boyle's  law,  has  doubled  its  volume. 
The  volume  of  the  air  displaced  is  therefore  twice  as  great  :  but  since 
its  density  has  become  only  one-half,  the  weight,  and  consequently 


-153] 


Balloons. 


147 


the  upward  buoyancy,  are  the  same.  When  once  the  balloon  is 
completely  dilated,  if  it  continue  to  rise,  the  force  of  the  ascent  de- 
creases, for  the  volume  of  the  displaced  air  remains  the  same, 
but  its  density  diminishes,  and 
a  time  arrives  at  which  the  buoy- 
ancy is  only  equal  to  the  weight 
of  the  balloon.  The  balloon  can 
now  only  take  a  horizontal  direc- 
tion, carried  by  the  currents  of 
air  which  prevail  in  the  atmo- 
sphere. The  aeronaut  knows  by 
the  barometer  whether  he  is 
ascending  or  descending  ;  and 
by  the  same  means  he  deter- 
mines the  height  which  he  has 
reached.  A  long  flag  fixed  to 
the  boat  would  indicate,  by  the 
position  it  takes  either  above  or 
below,  whether  the  balloon  is 
descending  or  ascending. 

When  the  aeronaut  wishes  to 
descend,  he  opens  the  valve  at 
the  top  of  the  balloon  by  means 
of  the  cord,  which  allows  gas  to 
escape,  and  the  balloon  sinks. 
If  he  wants  to  descend  more 
slowly,  or  to  rise  again,  he 
empties  out  bags  of  sand,  of 
which  there  is  an  ample  supply 
in  the  car.  The  descent  is 
facilitated  by  means  of  a  grap- 
pling iron  fixed  to  the  boat. 
When  once  this  is  fixed  to  any 
obstacle,  the  balloon  is  lowered 
by  pulling  the  cord. 

The  only  practical  applica- 
tions which  air  balloons  have 


Fig.  136. 


hitherto  had,  have  been  in  military  reconnoitring.  At  the  battle  of 
Fleurus  in  1794,  a  captive  balloon,  that  is,  one  held  by  a  cord,  was 
used,  in  which  there  was  an  observer  who  reported  the  movements  of 
the  enemy  by  means  of  signals.  At  the  battle  of  Solferino  the  move- 


148 


On  Gtises. 


[153- 


ments  and  dispositions  of  the  Austrian  troops  were  watched  by  a 
captive  balloon;  and  in  the  war  in  America  balloons  were  frequently 
used  ;  while  the  part  which  they  played  in  the  siege  of  Paris  is  still 
fresh  in  all  memories.  Many  ascents  have  recently  been  made  by 
Mr.  Glaisher  for  the  purpose  of  making  meteorological  observations 
in  the  higher  regions  of  the  atmosphere  (152).  Air  balloons  can 
only  be  truly  useful  when  they  can  be  guided,  and  as  yet  all  attempts 
made  with  this  view  have  completely  failed.  There  is  no  other 
course  at  present  than  to  rise  in  the  air  until  there  is  a  current 
which  has  mere  or  less  the  desired  direction. 

154.  Parachute. — The  object  of  the  parachute  is  to  allow  an 
aeronaut  to  leave  the  balloon,  by  giving  him  the  means  of  lessening 


Fig.  137- 

the  rapidity  of  his  descent.  It  consists  of  a  large  circular  piece  of 
cloth  (fig.  137)  about  16  feet  in  diameter,  and  which  by  the  resist- 
ance of  the  air  spreads  out  like  a  gigantic  umbrella.  In  the 
centre  there  is  an  aperture,  through  which  the  air,  compressed  by 


-155] 


Bellows. 


149 


the  rapidity  of  the  descent,  makes  its  escape  ;  for  otherwise  oscilla- 
tions might  be  produced,  which,  when  communicated  to  the  boat, 
would  be  dangerous. 

In  fig.  136  there  is  a  parachute  attached  to  the  network  of  the 
balloon  by  means  of  a  cord,  which  passes  round  a  pulley,  and  is 
fixed  at  the  other  end  to  the  boat.  When  the  cord  is  cut,  the  para- 
chute sinks,  at  first  very  rapidly,  but  more  slowly  as  it  becomes 
distended,  as  represented  in  the  figure. 

155.  Bellows. — Fig.  138  represents  a  simple  form  of  bellows, 
the  action  of  which  depends  on  the  compression  of  air.  Between 
two  round  wooden  boards  provided  with  handles,  one  of  which  has 
a  valve  k,  while  the  other  works  on  a  hinge,  a  folded  leathern 


Fig.  138. 

bag  is  fastened.  The  inner  space  terminates  in  front  in  a  shorter 
iron  or  brass  pipe  d,  which  is  called  the  nozzle.  If  now  the  upper 
lid  is  raised,  the  valve  is  open,  the  space  is  quite  filled  with  air, 
and  when  it  is  compressed  it  flows  out  through  the  nozzle.  Thus 
this  arrangement,  like  that  of  the  force  pump,  produces  an  inter- 
mittent flow  of  air. 


150  Acoustics,  [156- 


BOOK   IV. 

ACOUSTICS. 

CHAPTER    I. 
PRODUCTION,  PROPAGATION,   AND   REFLECTION   OF   SOUND. 

1 56.  Province  of  acoustics. — The  study  of  sounds,  and  that  of 
the  vibrations  of  elastic  bodies,  form  the  province  of  acoustics. 

Music  considers  sounds  with  reference  to  the  pleasurable  feelings 
they  are  calculated  to  excite.  Acoustics  is  concerned  with  the 
questions  of  the  production,  transmission,  and  comparison  of 
sounds.  To  this  may  be  added  the  physiological  question  of  the 
perception  of  sounds. 

Sound  is  a  peculiar  sensation  excited  in  the  organ  of  hearing  by 
the  vibratory  motion  of  bodies,  when  this  motion  is  transmitted  to 
the  ear  through  an  elastic  medium. 

Take  for  instance  the  string  of  a  musical  instrument,  when  it  is 
pulled  or  sounded  by  a  bow  (fig.  139).  When  this  is  pulled  aside 
from  the  position  acb,  where  it  is  at  rest,  to  the  position  adb,  all  the 


Fig.  139- 

points  being  more  or  less  out  of  their  position  of  equilibrium,  when 
the  string  is  left  to  itself,  it  tends  to  revert  to  its  original  position 
acb,  owing  to  its  elasticity.  In  virtue,  however,  of  its  acquired  ve- 
locity, it  passes  beyond  it  as  far  as  aeb,  all  the  points  being  then 
virtually  as  far  out  of  their  position  of  rest  as  they  were  at  adb. 
But  as  the  elasticity  still  continues  to  art,  not  merely  does  the  string 
revert  to  its  original  position,  but  it  again  passes  beyond  it,  and  so 


••¥ 

-157]  Sound  Waves.  1 5 1 

on,  the  amplitude  of  its  path  becoming  smaller  and  smaller,  as  re- 
presented by  the  dotted  lines  in  the  figure,  until  it  ultimately  reverts 
to  its  original  state  of  equilibrium.  Hence  each  point  of  the  string 
makes  a  backward  and  forward,  or  vibratory  motion,  like  that  of  the 
pendulum.  The  passage  from  the  position  adb  to  aeb,  and  back  to 
adb)  is  called  a  complete  vibration  or  oscillation ;  the  passage  from 
adb  to  aeb,  or  from  aeb  to  adb,  is  a  semi-vibration  or  semi-oscillation. 

Any  body  which  vibrates  or  yields  a  sound,  is  called  a  sonorous 
or  sounding  body.  The  vibrations  of  sounding  bodies  are  generally 
too  rapid  to  be  counted  or  even  distinctly  seen.  Yet  they  may  be 
rendered  evident  in  a  variety  of  ways.  .  Thus,  if  a  tolerably  large 
bell  jar  be  made  to  sound  by  striking  it  with  the  finger,  and  a  small 
ivory  bell  suspended  by  a  thread  be  approached  to  it,  the  ball  will 
be  observed  to  receive  a  series  of  rapid  shocks  from  the  sides  of  the 
bell,  showing  that  it  is  in  a  state  of  vibration.  Or,  if  a  plate  of 
metal  be  fixed  horizontally  at  one  end,  and  sand  be  strewed  over 
it,  when  the  plate  is  made  to  vibrate  by  briskly  moving  a  violin  bow 
against  the  edge,  the  sand  becomes  violently  agitated,  which  is  ob- 
viously due  to  the  vibrations  of  the  plate. 

157.  Propagation  of  sound  in  the  air.  Sound  waves. — 
After  having  ascertained  that  when  a  body  emits  a  sound,  its  mole- 
cules are  in  a  state  of  vibration,  it  remains  to  explain  how  these  are 
transmitted  to  the  ear  to  produce  the  sensation  of  sound.  Sound 
always  requires  for  its  transmission  an  elastic  medium,  which  at 
one  end  is  in  contact  with  the  sounding  body,  and  at  the  other  with 
the  organ  of  hearing.  Air  is  the  ordinary  medium  through 'which 
sound  is  transmitted.  As  air  is  very  mobile,  compressible,  and 
elastic,  its  molecules  being  in  contact  with  different  points  of  the 
sounding  body  acquire  movements  which  are  similar  to  those  of 
these  points  ;  they  go  and  come  with  these  points,  so  that  each 
molecule  of  air  in  contact  with  the  body  is  pushed  forward  by  it  in 
the  direction  of  the  sound,  and  returns,  having  communicated  its 
motion  to  the  next  molecule ;  this  then  acts  in  the  same  manner  on 
the  next  molecule,  and  so  on  to  the  molecules  in  contact  with  the 
tympanum  or  drum.  This  is  the  name  given  to  a  membrane  placed 
at  the  end  of  the  auditory  canal  of  the  ear;  it  receives  the  vibrations 
of  the  air,  which  it  transmits  by  a  series  of  small  bones  and  liquids 
to  the  acoustic  nerve,  and  thence  to  the  brain,  which  finally  per- 
ceives the  sensation  of  sound. 

At  each  impulse  imparted  by  a  sounding  body  to  the  molecules 
of  air  in  contact  with  it,  these  molecules  pressing  in  turn  upon  the 


152 


A  cons  tics. 


[157 


succeeding  ones,  a  condensed  part  is  produced  in  the  air  to  a  certain 
distance  which  is  called  the  condensed  ivave ;  then,  when  the  vibra- 
ting body  reverts  to  its  original  position,  the  molecules  nearest  to 
it  follow  it  in  its  motion,  so  that  there  is  formed  in  the  air  a  rarefied 
part  which  follows  the  condensed  wave,  and  which  is  called  the 
rarefied  'wave.  A  condensed  and  a  rarefied  wave  together  form  a 
sound  wave.  A  sounding  body  is  a  centre  from  which  these  waves 
are  emitted  all  round  it  in  the  form  of  continually  increasing  spheres, 
and  thus  it  is  that  sound  is  propagated  by  a  body  in  all  directions. 
Fig.  140  furnishes  a  rough  illustration  of  this  process.  If  a  stone  is 
thrown  into  still  water,  there  are  formed  round  the  point  where  it 


rig.  140. 

falls  a  series  of  concentric  waves,  which  continually  increase,  and 
which  give  an  idea  of  the  propagation  of  sound  waves  in  the  air. 

In  the  case  of  very  intense  sounds,  the  disturbance  communicated 
to  the  air  in  the  form  of  sound  waves  may  be  very  considerable. 
Thus  the  waves  produced  by  thunder,  by  the  report  of  cannon,  and 
by  gunpowder  explosion,  are  frequently  powerful  enough  to  break 
whole  panes  of  glass. 

158.  Coexistence  of  sound  waves. — It  is  to  be  observed  that 
several  sounds  may  be  propagated  in  air  without  destroying  each 
other.  Thus  in  the  most  complicated  orchestral  music,  a  person 
with  a  practised  ear  can  readily  follow  the  sound  of  each  instru- 
ment. Yet  a  loud  sound  interferes  with  a  weak  one  ;  thus  the  sound 
of  a  drum  overpowers  the  human  voice.  Sounds  also  which  are 
too  weak  to  be  distinctly  heard,  accumulate  upon  each  other,  and 
produce  a  confused  sound,  which  becomes  perceptible  to  the  ear. 


- 161]  Propagation  of  Sound  in  Liquids  and  Solids.    153 

Such  is  the  cause  of  the  murmuring  of  water,  the  rustling  of  leaves 
in  woods,  and  the  dashing  of  waves  against  the  shores. 

159.  Sound  is  not  propagated  in  vacuo. — The  vibrations  of 
elastic  bodies  can  only  produce  the  sensation  of  sound  in  us,  by  the 
intervention  of  a  medium  interposed  between  the  ear  and  the  so- 
norous body,  and  vibrating  with  it.     This  medium  is  usually  the 
air,  but  all  gases,  vapours,  liquids,  and  solids  also  transmit  sound. 

The  following  experiment  shows  that  the  presence  of  a  ponder- 
able medium  is  necessary  for  the  propagation  of  sound.  A  tolerably 
large  glass  globe  (fig.  141),  provided  with  a  stop- 
cock, has  a  small  bell  suspended  in  the  interior 
by  a  thread.  A  vacuum  having  been  created  in 
the  globe  by  means  of  the  air-pump,  no  sound  is 
emitted  when  the  globe  is  shaken,  though  the 
clapper  may  be  seen  to  strike  against  the  bell ; 
but  if  air,  or  any  other  gas  or  vapour,  be  admitted, 
sound  is  distinctly  heard  each  time  the  globe  is 
agitated. 

The  experiment  may  also  be  made  by  placing 
a  small  metallic  bell,  which  is  continually  struck  g' I4It 

by  a  small  hammer  by  means  of  clockwork,  or  an  ordinary  musi- 
cal box,  under  the  receiver  of  the  air-pump.  As  long  as  the  re- 
ceiver is  full  of  air  at  the  ordinary  pressure,  the  sound  is  trans- 
mitted ;  but,  in  proportion  as  the  air  is  exhausted,  the  sound 
becomes  feebler,  and  is  imperceptible  in  a  vacuum. 

To  ensure  the  success  of  the  experiment,  the  bellwork  or  musical 
box  must  be  placed  on  wadding  ;  for  otherwise  the  vibrations 
would  be  transmitted  to  the  air  through  the  plate  of  the  machine. 

1 60.  Propagation  of  sound  in  liquids  and  solids. — Sound  is 
also  propagated  in  liquids.     When  two  bodies  strike  against  each 
other  under  water,  the  shock  is  distinctly  heard  ;  and  a  diver  at  the 
bottom  of  the  water  can  hear  the  sound  of  voices  on  the  bank. 

The  conductibility  of  solids  is  such,  that  the  scratching  of  a  pen 
at  the  end  of  a  long  wooden  rod  is  heard  at  the  other  end.  In  like 
manner  if  a  person  speaks  with  a  low  voice  at  the  end  of  a  pine  rod, 
25  to  30  yards  long,  he  is  heard  by  a  person  whose  ear  is  applied 
against  the  other  end,  while  a  person  who  is  near,  hears  nothing. 
The  earth  conducts  sound  so  well,  that  at  night,  when  the  ear  is 
applied  to  the  ground,  the  steps  of  horses  or  any  other  noise  at  great 
distances  is  heard. 

Jo i.  Velocity  of  sound  in  air. — Numerous  phenomena  show 


154  Acoustics.  [16  i- 

that  sound  requires  a  certain  time  to  pass  from  one  place  to  another. 
Thus,  if  we  pay  attention  to  a  woodmen  felling  trees  at  a  distance, 
we  see  the  axe  fall  in  silence,  and  only  hear  the  sound  a  moment 
afterwards.  In  like  manner,  when  a  gun  is  fired,  the  report  is  heard 
after  the  flash  of  light.  Thunder,  too,  is  only  heard  some  time 
after  lightning,  although  in  the  cloud  both  thunder  and  lightning  are 
produced  simultaneously. 

The  velocity  of  sound  was  determined  experimentally  by  the 
members  of  the  Bureau  of  Longitude  of  Paris  in  June  1822,  during 
the  night.  A  cannon  was  placed  on  a  hill  at  Montlhery  near  Paris, 
and  another  on  a  plateau  near  Villejuif.  The  distance  of  the  two 
places  was  carefully  measured,  and  was  found  to  be  61,045  feet,  and 
a  gun  was  fired  at  each  station  twelve  successive  times  at  intervals 
of  10  minutes  (fig.  142).  Observers  placed  near  the  guns  noted 


Fig.  142. 

by  means  of  accurate  and  delicate  watches,  the  time  which  elapsed 
between  the  appearance  of  the  flash  and  the  hearing  the  sound  at 
the  opposite  station ;  and  the  mean  of  the  observations  gave  the 
number  54-6  seconds.  This  was  just  the  time  which  the  sound  re- 
quired to  travel  from  one  station  to  the  other ;  for  we  shall  after- 
wards see  (308)  that  the  velocity  of  light  is  such  that  the  time  it 
requires  to  traverse  the  above  distance  is  inappreciable.  Hence 
by  a  simple  calculation  we  find  that  sound  travels  1,118  feet  in  a 
second. 


-162]  Velocity  of  Sound.  155 

The  above  observations  were  made  when  the  air  was  at  a  tem- 
perature of  1 6°.  At  a  lower  temperature  the  velocity  of  sound  is 
less. 

From  some  accurate  experiments  made  by  the  above  method 
near  Amsterdam,  the  velocity  of  sound  is  taken  at  1,093  feet  Per 
second  in  dry  air  at  zero.  Its  velocity  increases  about  2  feet  per 
second  for  every  degree  centigrade.  So  that  at  15°  C.,  which  is 
the  ordinary  temperature,  the  velocity  of  sound  is  1,120  feet  per 
second. 

A  knowledge  of  the  velocity  of  sound  enables  us  to  measure 
distances.  Thus,  suppose  we  want  to  known  the  distance  at  which 
a  gun  is  fired,  of  which  we  only  hear  the  report  1 5  seconds  after 
seeing  the  flash.  As  sound  travels  at  1,120  feet  in  a  second,  it 
must  traverse  16,800  feet  in  the  time  mentioned,  and  this  would  be 
the  distance  at  which  the  gun  was  fired.  In  the  same  manner  we 
may  calculate  the  depth  of  a  well  from  the  number  of  seconds 
which  elapses  between  the  moment  at  which  a  stone  falls  into  it  and 
that  at  which  the  sound  is  produced.  The  calculation  is,  however, 
more  complicated,  for  the  time  which  the  body  requires  in  falling 
has  to  be  taken  into  account. 

The  velocity  of  sound  is  not  the  same  in  different  gases  ;  it  is 
greater  in  those  which  are  less  dense.  Dulong  found  the  velocity 
at  zero  to  be  846  feet  per  second  for  carbonic  acid,  1,040  feet  in 
oxygen,  and  1,093  in  air,  1,106  in  carbonic  oxide,  and  4,163  feet  in 
hydrogen. 

The  velocity  of  sound  is  the  same  in  air  for  all  sounds,  whether 
strong  or  weak,  grave  or  acute. 

For  this  reason  the  tune  played  by  a  band  is  heard  at  a  great 
distance  without  alteration,  except  in  intensity,  which  could  not  be 
the  case  if  some  sounds  travelled  more  rapidly  than  others. 

162.  Velocity  of  sound  in  liquids  and  in  solids. — We  have 
already  seen  that  liquids  conduct  sound ;  they  even  conduct  it 
better  than  gases.  The  velocity  of  sound  in  water  was  investi- 
gated in  1827  by  Colladon  and  Sturm.  They  moored  two  boats, 
fig.  143,  at  a  known  distance  in  the  Lake  of  Geneva.  The  first, 
B,  supported  a  bell  C  immersed  in  water,  and  a  bent  lever  provided 
at  one  end  with  a  hammer  b  which  struck  the  bell,  and  at  the 
other  with  a  lighted  wick  e,  so  arranged  that  it  ignited  some 
powder  m,  the  moment  the  hammer  struck  the  bell.  To  the  second 
boat  was  affixed  an  ear- trumpet,  the  bell,  gfk,  of  which  was  in 
water,  while  the  mouth  o,  was  applied  to  the  ear  of  the  observer, 


Acoustics. 


[162- 


so  that  he  could  measure  the  time  between  the  flash  of  light,  and 
the  arrival  of  sound  by  the  water.  By  this  method  the  velocity  was 
found  to  be  4,708  feet  in  a  second  at  the  temperature  8°,  or  four 
times  as  great  as  in  air. 


That  sound  travels  more  rapidly  in  solids  than  in  air  is  easily 
shown.  If  a  person  holds  his  ear  against  one  end  of  a  tolerably 
long  iron  bar,  while  another  person  gives  a  hard  blow  at  the  other 
end,  two  distinct  sounds  are  heard  ;  the  first  transmitted  by  the 
metal,  and  the  other  transmitted  by  the  air.  The  velocity  of  sound 
in  iron  is  16,800  feet  in  a  second;  in  copper,  11,600;  in  oak, 
10,900  ;  and  in  fir,  15,220  feet. 

163.  Reflection  of  sound. — We  have  seen  that  sound  is  propa- 
gated in  air  by  means  of  spherical  waves,  alternately  condensed 
and  rarefied,  and  which  are  developed  about  it  in  all  directions. 
So  long  as  these  sound  waves  are  not  obstructed  in  their  motion, 
they  are  propagated  in  the  form  of  concentric  spheres  ;  but  when 
they  meet  with  an  obstacle,  they  follow  the  general  law  of  elastic 
bodies  ;  that  is,  they  are  repelled  like  an  ivory  ball,  which  strikes 
against  a  wall ;  they  return  upon  themselves,  forming  new  concentric 
waves,  which  seem  to  emanate  from  a  second  centre  on  the  other 
side  of  the  obstacle.  This  phenomena  constitutes  the  reflection  oj 
sound. 

The  reflection  of  sound,  or  rather  of  sound  waves,  follows  the 


-164] 


Reflection  of  Sound. 


same  laws  as  the  reflection  of  heat  and  ot  light,  which  we  shall 
subsequently  have  to  explain. 

The  reflection  of  sound  may  be  demonstrated  by  means  of  the 
arrangement  represented  in  fig.  144,  which  are  two  parabolic 
mirrors  placed  at  some  distance  opposite  each  other.  At  a  certain 
position  in  front  of  one  of  them,  called  the  focus,  is  placed  a  watch 
or  other  convenient  sounding  body.  It  is  a  property  of  this  posi- 


Fig.  144. 

tion,  the  focus,  that  all  sound  waves  emanating  from  it  which  fall 
on  the  adjacent  mirror  are  thrown  back  in  parallel  rays.  If  these 
parallel  rays  fall  on  the  second  mirror  they  are  reflected  to  its 
focus,  so  that  if  an  ear  be  placed  there  the  sound  waves  are  con- 
centrated in  the  ear,  and  the  ticking  of  the  watch  is  distinctly 
heard,  which  is  not  the  case  if  the  ear  is  not  in  this  position. 

164.  Echoes  and  resonances. — An  echo  is  the  repetition  of  a 
sound  in  the  air,  caused  by  its  reflection  from  some  more  or  less 
distant  obstacle.  Thus,  if  a  few  words  are  loudly  spoken  at  a  cer- 
tain distance  from  a  wood,  a  rock,  or  a  building,  it  usually  happens 
that,  after  a  brief  interval,  the  same  phrase  is  heard  repeated,  as 
if  spoken  in  the  distance  by  another  person  ;  these  are  the  sound 
waves,  which  are  reflected  by  the  obstacle.  There  must,  however, 
be  a  certain  distance  between  the  place  at  which  the  sound  is  pro- 
duced and  that  at  which  it  is  heard. 

A  very  sharp  quick  sound  can  produce  an  echo  when  the  re- 
flecting surface  is  55  feet  distant  ;  but  for  articulate  sounds  at  least 
double  that  distance  is  necessary,  for  it  may  be  easily  shown  that 
no  one  can  pronounce  or  hear  distinctly  more  than  five  syllables  in 
a  second.  Now,  as  the  velocity  of  sound  at  ordinary  temperatures 


158  A  coustics.  [164- 

may  be  taken  at  1,120  feet  in  a  second,  in  a  fifth  of  that  time  sound 
would  travel  224.  feet.  If  the  reflecting  surface  is  112  feet  distant, 
sound  would  travel  through  224  feet  in  going  and  returning.  The 
time  which  elapses  between  the  articulated  and  the  reflected  sound 
would,  therefore,  be  a  fifth  of  a  second,  the  two  sounds  would  not 
interfere,  and  the  reflected  sound  would  be  distinctly  heard.  A 
person  speaking  with  a  loud  voice  in  front  of  a  reflecting  surface 
at  the  distance  of  112  feet  can  only  distinguish  the  last  reflected 
syllable  :  such  an  echo  is  said  to  be  monosyllabic.  If  the  reflector 
were  at  a  distance  of  two  or  three  times  1 1 2  feet,  the  echo  would 
be  dissyllabic,  trisyllabic,  and  so  on. 

Multiple  echoes  are  those  which  repeat  the  same  sound  several 
times ;  this  is  the  case  when  two  opposite  surfaces  (for  example, 
two  parallel  walls)  successively  reflect  sound.  There  are  echoes 
which  repeat  the  same  sound'  20  or  30  times.  An  echo  in  the 
chateau  of  Simonetta  in  Italy,  repeats  a  sound  30  times.  At 
Woodstock  there  is  one  which  repeats  from  1 7  to  20  syllables. 
Near  Verdun  is  an  echo  formed  by  two  parallel  towers,  at  a  dis- 
tance from  each  other  of  about  164  feet.  A  person  placing  himself 
between  them,  and  speaking  a  word  with  a  loud  voice,  hears  it 
repeated  a  dozen  times.  Echoes  usually  modify  sound  ;  some  re- 
peat it  with  noise  ;  others  with  a  mocking,  laughing  tone,  or  a 
plaintive  accent. 

We  have  seen  that  when  the  distance  at  which  a  sound  is  re- 
flected is  112  feet  an  echo  is  produced  ;  and  the  question  may  be 
asked,  what  happens  when  the  distance  is  less  than  this  ?  As,  the 
sound  has  then  a  smaller  distance  to  traverse,  both  in  going  and 
coming,  than  112  feet,  it  follows  that  the  reflected  sound  is  added 
to  the  directly  spoken  one.  They  cannot  be  heard  separately,  but 
the  sound  is  strengthened.  This  is  what  is  called  resonance,  and 
its  effects  are  so  much  the  more  marked  the  more  elastic  are  the 
surfaces  from  which  the  sound  is  reflected.  In  racket  courts  and 
in  uninhabited  houses,  where  there  is  no  furniture,  the  walls,  the 
flooring,  and  the  ceiling  readily  vibrate,  and  we  all  know  how  the 
noise  of  footsteps  and  the  sound  of  the  voice  then  resound.  Tapestry 
and  hangings,  which  are  not  elastic,  deaden  the  sound. 

As  the  laws  of  the  reflection  of  sound  are  the  same  as  those  of 
light  and  heat,  curved  surfaces  of  natural  occurrence  often  produce 
acoustic  foci,  like  the  luminous  and  calorific  foci  produced  by 
mirrors.  If  a  person  standing  under  the  arch  of  a  bridge  speaks 
with  his  face  turned  towards  one  of  the  piers,  the  sound  is  repro- 


-165]   Cattses  which  influence  the  Intensity  of  Sound.    1 59 

duced  near  the  other  pier  with  such  distinctness  that  a  conversation 
can  be  kept  up  in  a  low  tone,  which  is  not  heard  by  any  one 
standing  in  the  intermediate  spaces. 

There  is  a  square  room  with  an  elliptical  ceiling,  on  the  ground- 
floor  of  the  Conservatoire  des  Arts  et  Metiers,  in  Paris,  wjiich  pre- 
sents this  phenomenon  in  a  remarkable  degree  when  persons  stand 
in  the  two  foci  of  the  ellipse.  So  also  bellying  sails  act  as  mirrors 
on  sound. 

It  is  not  merely  by  solid  surfaces,  such  as  walls,  rocks,  etc., 
that  sound  is  reflected.  It  is  also  reflected  by  clouds,  and  on 
passing  into  a  layer  of  air  of  greater  density  than  its  own  ;  and  is 
said  to  be  reflected  by  the  vesicles  of  mist. 

Whispering  galleries  are  formed  of  smooth  walls,  having  a  con- 
tinuous curved  form.  The  mouth  of  the  speaker  is  presented  at 
one  point,  and  the  ear  of  the  hearer  at  another  and  distant  point. 
In  this  case,  the  sound  is  successively  reflected  from  one  point  to 
the  other  until  it  reaches  the  ear. 

Different  parts  of  the  earth's  surface  are  unequally  heated  by 
the  sun,  owing  to  the  shadows  of  trees,  evaporation  of  water,  and 
other  causes,  so  that  in  the  atmosphere  there  are  numerous  ascend- 
ing and  descending  currents  of  air  of  different  densities.  Whenever 
a  sound  wave  passes  from  a  medium  of  one  density  into  another  it 
undergoes  partial  reflection  ;  in  some  cases  is  strong  enough  to 
form  an  echo,  and  always  distinctly  weakens  the  direct  sound. 
This  is  doubtless  the  reason,  as  Humboldt  remarks,  why  sound 
travels  further  at  night  than  at  daytime  ;  even  in  the  South  Ameri- 
can forests,  where  the  animals,  which  are  silent  by  day,  fill  the 
atmosphere  in  the  night  with  thousands  of  confused  sounds. 

165.  Causes  which  influence  the  intensity  of  sound. — Many 
causes  modify  the  force  or  the  intensity  of  the  sound.  These  are, 
the  distance  of  the  sonorous  body,  the  amplitude  of  the  vibrations, 
the  density  of  the  air  at  the  place  where  the  sound  is  produced, 
the  direction  of  the  currents  of  air,  and,  lastly,  the  nearness  of  other 
bodies  which  can  enter  into  a  state  of  vibration. 

i.  The  intensity  of  sound  is  inversely  as  the  square  of  the  distance 
of  the  sounding  body  from  the  ear.  This  law  has  been  deduced  by 
calculation,  but  it  may  be  also  demonstrated  experimentally.  Let 
us  suppose  several  sounds  of  equal  intensity,  for  instance,  bells  of 
the  same  kind,  struck  by  hammers  of  the  same  weight,  falling  from 
equal  heights.  If  four  of  these  bells  are  placed  at  a  distance  of 
20  yards  from  the  ear,  and  one  at  a  distance  of  10  yards,  it  is 


160  Acoustics.  [165- 

found  that  the  single  bell  produces  a  sound  of  the  same  intensity 
as  the  four  bells  struck  simultaneously.  Consequently,  for  double 
the  distance,  the  intensity  of  the  sound  is  only  one-fourth. 

The  distance  at  which  sounds  can  be  heard  depends  on  their 
intensity.  The  report  of  a  volcano  at  St.  Vincent  was  heard  at 
Demerara,  300  miles  off,  and  the  firing  at  Waterloo  was  heard  at 
Dover. 

ii.  The  intensity  of  the  sound  increases  -with  the  amplitude  of 
the  vibrations  of  the  sounding  body.  The  connection  between  the 
intensity  of  the  sound  and  the  amplitude  of  the  vibrations,  is 
readily  observed  by  means  of  vibrating  cords.  For  if  the  cords 
are  somewhat  long  the  oscillations  are  perceptible  to  the  eye,  and 
it  is  seen  that  the  sound  is  feebler  in  proportion  as  the  amplitude 
of  the  oscillations  decreases. 

For  the  same  reason  the  dying  sound  of  the  last  blows  of  a 
bell  become  gradually  feebler,  until  they  are  ultimately  extin- 
guished. 

iii.  The  intensity  of  sound  depends  on  the  density  of  the  air  in 
the  place  in  which  it  is  produced.  As  we  have  already  seen  (159), 
when  an  alarum  moved  by  clockwork  is  placed  under  the  bell-jar 
of  the  air-pump,  the  sound  becomes  weaker  in  proportion  as  the 
air  is  rarefied. 

In  hydrogen,  which  has  about  T\th  the  density  of  air,  sounds  are 
much  feebler,  although  the  pressure  is  the  same.  In  carbonic  acid, 
on  the  contrary,  which  is  half  as  heavy  again  as  air,  sounds  are 
more  intense.  On  high  mountains,  where  the  air  is  much  rarefied, 
it  is  necessary  to  speak  with  some  effort  in  order  to  be  heard,  and 
the  discharge  of  a  gun  produces  only  a  feeble  sound.  During  a 
severe  frost,  sounds  are  heard  at  a  greater  distance,  because  air  is 
then  more  dense  and  more  homogeneous  ;  and  country  people  will 
often  predict  the  weather  from  the  sound  of  the  village  bell.  For 
the  propagation  of  the  sound  is  modified  by  the  presence  of 
moisture,  which  alters  the  elasticity  and  the  density. 

iv.  The  intensity  of  soimd  is  modified  by 'the  motion  of  the  atmo- 
sphere and  the  direction  of  the  wind.  In  calm  weather  sound  is 
always  better  propagated  than  when  there  is  wind  ;  in  the  latter 
case,  for  an  equal  distance,  sound  is  more  intense  in  the  direction 
of  the  wind  than  in  the  contrary  direction. 

v.  Lastly,  sound  is  strengthened  by  the  nearness  of  a  sounding 
body.  A  string  made  to  vibrate  in  free  air  and  not  near  a  sounding 
body  has  but  a  very  feeble  sound ;  but  when  it  vibrates  above  a 


-166] 


Speaking  Tubes. 


161 


sounding-box,  as  in  the  case  of  the  violin,  guitar,  or  violoncello, 
its  sound  is  much  more  intense.  This  arises  from  the  fact  that 
the  'box  and  the  air  which  it  contains  vibrate  in  unison  with  the 
string.  Hence  the  use  of  sounding-boxes  in  stringed  instruments. 
1 66.  Influence  of  tubes  on  the  transmission  of  sound. — The 
diminution  in  the  intensity  of  sound  with  the  distance  is  due  to  the 
fact,  that  the  sound  waves  are  propagated  in  the  form  of  continually 
increasing  spheres  ;  and  it  may  indeed  be  proved  geometrically, 
that  since  sound  is  thus  transmitted,  its  intensity  must  be  inversely 


,••'''••'    ! 


Fig-  i45- 

as  the  square  of  the  distance  If,  however,  the  sound  is  sent 
through  a  long  tube,  the  waves  are  propagated  in  only  one  direction, 
and  sound  can  be  transmitted  to  great  distances  without  appreci- 
able alteration.  M.  Biot  found  that  in  one  of  the  Paris  water-pipes, 
1,040  yards  long,  the  voice  lost  so  little  of  its  intensity,  that  a 
conversation  could  be  kept  up  at  the  ends  of  the  tube  in  a  very 
low  tone  ;  so  much  so,  that  in  order  not  to  be  heard,  it  was  necessary, 
as  Biot  expressed  it,  not  to  speak,  at  all.  The  weakening  of  sound 
becomes,  however,  perceptible  in  tubes  of  large  diameter,  or  where 
the  sides  are  rough. 

This  property  of  transmitting  sounds  was  first  applied  in  Eng- 

M 


1 62 


Acoustics. 


[166- 


land  for  speaking  tubes,  which  are  used  in  hotels  and  large  estab- 
lishments for  transmitting  orders.  They  consist  of  caoutchouc 
tubes  of  small  diameter,  provided  at  each  end  with  an  ivory  or 
bone  mouthpiece,  and  passing  from  one  room  to  another.  If  a 
person  speaks  at  one  end  of  the  tube,  he  is  distinctly  heard  by  a 
person  applying  his  ear  (fig.  145)  at  the  other  end. 

One  of  the  most  important  applications  of  acoustical  principles 
is  the  Stethoscope.  It  consists  of  a  cylinder  of  hard  wood  about  a 
foot  long  and  i|  inch  broad  at  one  end,  and  in  which  a  longitudinal 
passage  is  bored.  One  end  of  the  stethoscope  is  held  against  the 
diseased  part  of  the  body,  and  the  ear  is  held  against  the  other.  The 
practised  physician  can  detect  the  existence  of  internal  cavities 
by  the  peculiar  sound  emitted,  and  which  is  strengthened  by  re- 
sonance. 

167.  Speaking:  trumpet. — These  instruments  are  based  both  on 
the  reflection  of  sound,  and  on  its  conductibility  in  tubes. 


Fig.  146. 

The  speaking  trumpet  as  its  name  implies,  is  used  to  render 
the  voice  audible  at  great  distances.  It  consists  of  a  slightly 
conical  tin  or  brass  tube  (fig.  146),  very  much  wider  at  one  end 
(which  is  called  the  bell),  and  provided  with  a  mouthpiece  at  the 
other.  The  larger  the  dimensions  of  this  instrument  the  greater 
is  the  distance  at  which  the  voice  is  heard.  Its  action  is  usually 
ascribed  to  the  successive  reflections  of  sound  waves  from  the 


-169]  Ear  Trumpet.  163 

sides  of  the  tube,  by  which  the  waves  tend  more  and  more  to  pass 
in  a  direction  parallel  to  the  axis  of  the  instrument.  By  means 
of  the  speaking  trumpet,  the  word  of  command  can  be  heard  on 
board  ship  above  the  noise  of  the  waves. 

1 68.  Ear  trumpet. — The  ear  trumpet  is  used  by  persons  who 
are  hard  of  hearing.  It  is  essentially  an  inverted  speaking  trumpet, 
and  consists  of  a  conical  metallic  tube,  one  of  whose  extremities, 
terminating  in  a  bell,  receives  the  sound,  while  the  other  end  is 
introduced  into  the  ear  (fig.  147).  The  action  of  this  instrument  is 
the  reverse  of  that  of  the  speaking  trumpet.  The  bell  serves  as 
mouthpiece  ;  that  is,  it  receives  the  sounds  coming  from  the  mouth 
of  the  person  who  speaks.  These  sounds  are  transmitted  by  a  series 


of  reflections  to  the  interior  ot  the  trumpet,  so  that  the  waves 
which  would  become  greatly  developed,  are  concentrated  on  the 
hearing  apparatus,  and  produce  a  far  greater  effect  than  divergent 
waves  would  have  done. 

In  man  and  many  animals  the  outer  ear  is  a  trumpet  which 
receives  the  sound  waves.  In  some  animals  this  part  of  the  audi- 
tory apparatus  is  long  and  flexible,  so  that  the  animal  can  thus 
easily  recognise  the  direction  from  which  the  sound  proceeds. 


CHAPTER   II. 

MUSICAL  SOUND.      PHYSICAL  THEORY  OF  MUSIC. 

169.  Difference  between  musical  sounds  and  noise. — Sounds 
are  distinguished  from  noises.  Sound  properly  so  called,  or  musical 
sound,  is  that  which  produces  a  continuous  sensation,  and  the 
musical  value  of  which  can  be  determined.  The  only  condition 
necessary  for  producing  a  musical  sound  is  that  the  individual  im- 
pulses shall  succeed  each  other  with  sufficient  rapidity  at  equal 

M  2 


164  Acoustics.  [169- 

intervals  of  time.  Whatever  be  its  origin,  whether  it  be  the  ticks 
of  a  watch,  or  the  puffs  of  a  locomotive,  if  this  condition  be  fulfilled, 
the  coalescence  of  the  separate  impressions  produces  a  musical 
sound. 

On  the  other  hand,  noise  is  either  a  sound  of  too  short  a  dura- 
tion to  be  determined,  like  the  report  of  a  cannon,  or  else  it  is  a 
confused  mixture  of  many  discordant  sounds,  like  the  rolling  of 
thunder,  the  rattling  of  a  box  of  nails,  or  the  noise  of  the  waves. 
Nevertheless,  the  difference  between  sound  and  noise  is  by  no  means 
precise.  Savart  has  shown  that  there  are  relations  of  height  in  the 
case  of  noise,  as  well  as  in  that  of  sound,  and  there  are  said  to  be 
certain  ears  sufficiently  well  organised  to  determine  the  musical 
value  of  the  sound  produced  by  a  carriage  rolling  on  the  pavement. 

The  action  of  a  noise  upon  the  ear  has  been  compared  to  that 
of  a  flickering  light  upon  the  eye  ;  both  are  painful,  in  consequence 
of  the  sudden  and  abrupt  changes  which  they  impose  upon  their 
respective  nerves. 

170.  Characteristics  of  musical  sounds. — Musical  tones  have 
three  leading  qualities,  namely ///£#,  intensity,  and  timbre  or  colour. 

i.  The  pitch  or  height  of  a  musical  tone  is  determined  by  the 
number  of  vibrations  in  a  second  yielded  by  the  body  producing  the 
tone. 

ii.  The  intensity  or  loudness  of  the  tone  depends  on  the  extent  of 
the  vibrations.  It  is  greater  when  the  extent  is  greater,  and  less 
when  it  is  less.  It  is,  in  fact,  nearly  or  exactly  proportional  to  the 
square  of  the  extent  or  amplitude  of  the  vibrations  which  produce 
the  tone. 

iii.  The  timbre  is  that  peculiar  quality  of  tone  which  distinguishes 
a  note  when  sounded  on  one  instrument  from  the  same  note  when 
sounded  on  another.  Thus  when  the  C  of  the  treble  stave  is 
sounded  on  a  violin  and  on  a  flute,  the  two  notes  will  have  the 
same  pitch,  that  is,  are  produced  by  the  same  number  of  vibrations 
per  second,  and  they  may  have  the  same  intensity,  and  yet  the  two 
tones  will  have  very  distinct  qualities,  that  is,  their  timbre  is  dif- 
ferent (179). 

171.  Syren. — The   vibrations   of    any   sounding  body   are   so 
numerous  that  they  cannot  be  followed  by  the  eye  and  counted. 
Various  forms  of  apparatus  have  been  invented  for  the  purpose 
of  determining  the  number  of  vibrations  corresponding  to  particular 
notes.     Of  these,  the  one  represented  in  fig.  148,  is  given  as  being 
the  simplest  and  most  intelligible.     It  consists  of  a  circular  disc  of 


-172]  Syren.  165 

stout  cardboard  or  of  sheet  metal  about  a  foot  in  diameter.  This 
disc  is  perforated  by  four  concentric  series  of  small  equidistant  holes. 
For  simplicity's  sake  the  inner  of 
these  is  represented  as  having  1 2, 
the  second  15,  the  third  18,  and 
the  fourth  24  holes  ;  but  a  mul- 
tiple of  these  ratios,  say  48,  60, 
72,  and  96,  is  more  convenient. 

The  disc  is  made  to  rotate 
rapidly,  and  the  most  convenient 
plan  is  to  fix  it  on  a  turning  table 
(fig.  15),  in  the  place  of  AB. 
Then  by  means  of  a  glass  tube, 
drawn  out  at  one  end  so  as  to  be 
smaller  than  the  diameter  of  the 
holes ;  a  current  of  air  is  directed  Flg'  I48> 

against  one  of  the  series  of  holes  in  the  rotating  disc.  A  tone  is  now 
heard,  which  is  tolerably  pure  when  the  rotations  are  sufficiently 
rapid,  and  the  number  of  vibrations  of  which  can  be  readily  deter- 
mined. Suppose,  for  instance,  that  there  are  48  in  the  inner  series 
of  holes.  Then  each  time  a  hole  passes  in  front  of  the  glass  tube  a 
condensed  wave  is  produced  which  reaches  the  ear  in  the  ordinary 
manner.  If,  for  example,  the  disc  makes  16  turns  in  a  second,  in 
each  second  16  times  48  or  768  holes,  pass  in  front  of  the  tube,  and 
there  are  produced  768  waves,  which  fall  upon  the  ear  within  a 
second,  at  equal  intervals  of  time.  If  in  like  manner  the  tube  were 
held  over  the  second  series  of  holes,  while  the  rotation  goes  on  at 
the  same  rate,  we  should  hear  the  tones  corresponding  to  16  times 
60,  or  960  vibrations  in  a  second.  Thus  proceeding  in  like  manner, 
and  moving  the  tube  successively  from,  the  central  to  the  circum- 
ferential series  of  holes  we  hear  successively,  the  fundamental  note, 
the  major  third,  the  fifth,  and  the  octave  (173). 

172.  Limit  of  perceptible  sounds. — Savart,  a  French  physicist, 
was  the  first  to  determine  the  limit  of  the  number  of  vibrations 
which  the  ear  could  perceive.  He  invented  an  apparatus  for  this 
purpose  which  is  known  as  .  Savarfs  toothed  wheel.  It  consists 
essentially  of  a  metal  wheel  with  a  series  of  equidistant  sharp  teeth 
on  its  periphery.  This  is  made  to  rotate  at  a  uniform  rate,  and  a 
card,  or  still  better,  a  thin  elastic  steel  plate  is  held  so  that  in  the 
rotation  of  the  wheel  each  of  the  teeth  strikes  against  the  plate,  and 
each  time  produces  a  sound.  If,  for  instance,  the  rim  of  the  wheel 


1 66  Acoustics.  [172- 

has  6co  teeth,  and  it  is  made  to  rotate  4  times  in  a  second,  2400 
impulses  are  given  in  a  second.  The  number  of  impulses  depends 
thus  on  the  velocity  of  rotation  and  the  sounds  produced  are  pure 
and  continuous. 

Thus  to  determine  the  number  of  vibrations  corresponding  to 
any  particular  note,  it  is  simply  necessary  to  turn  the  wheel  at  a 
uniform  rate  until  it  produces  a  note  in  unison  (173)  with  the  one  in 
question.  Knowing  then  the  number  of  teeth  on  the  wheel  and  the 
rate  of  rotation,  the  number  of  vibrations  can  be  at  once  calculated. 
By  means  of  this  apparatus  Savart  ascertained  that  the  deepest 
sounds  are  produced  by  16  vibrations  in  a  second.  If  the  number 
of  vibrations  is  less,  no  sound  is  heard.  The  same  physicist 
found  that  the  highest  sound  which  the  ear  can  perceive  cor- 
responds to  48,000  vibrations  in  a  second.  Between  these  two 
limits  it  will  be  seen  what  an  enormous  quantity  of  sounds 
may  be  produced  and  perceived.  Yet  the  sounds  used  in  music, 
and  more  especially  in  singing,  are  comprised  within  much 
narrower  limits.  Thus  the  number  of  vibrations  produced  by  the 
human  voice  has  been  ascertained ;  and  it  has  been  found  that 
the  lowest  notes  of  a  man's  voice  are  made  by  190  vibrations  in 
a  second,  and  the  highest  notes  by  678.  The  lowest  note  of  a 
woman's  voice  corresponds  to  572  vibrations,  and  the  highest  to 
1,606. 

173.  Musical  scale.  Gamut. — The  human  ear  can  distin- 
guish among  several  sounds  not  merely  which  is  the  highest,  or  the 
lowest,  but  it  can  also  estimate  the  relations  which  exist  between 
the  numbers  of  vibrations  corresponding  to  each  of  these  sounds. 
Not,  indeed,  that  we  can  say  whether  one  sound  produces  two  or 
three  times  as  many  vibrations  as  another  ;  but  whenever  the 
number  of  vibrations  of  two  successive  or  simultaneous  sounds  are 
in  a  simple  ratio,  these  sounds  excite  in  us  an  agreeable  sensation, 
which  varies  with  the  ratio  of  the  vibrations  of  the  two  sounds,  and 
which  the  ear  can  readily  estimate.  Hence  results  a  series  of 
sounds  characterised  by  relations  which  have  their  origin  in  the 
nature  of  our  organisation,  and  which  constitute  what  is  called  the 
musical  scale. 

In  this  series  the  sounds  are  reproduced  in  the  same  order,  in 
periods  of  seven,  each  period  constituting  a  gamut ;  and  the  seven 
sounds  or  notes  of  each  gamut  are  designated  by  the  names  C,  D, 
E,  F,  G,  A,  B,  or  by  nt  or  do,  re,  mi,  fa,  sol,  la,  si.  The  first  six  of 
these  letters  are  the  first  syllables  of  the  lines  of  a  hymn  which 


-174]  Musical  Scale.  167 

was  sung  by  the  chorister  children  to  St.  John,  their  patron  saint, 
when  they  prayed  to  be  freed  from  hoarseness  ;  and  the  word  si  is 
formed  of  the  first  letters  of  St.  John's  name. 

"Ut  queant  laxis  resonare  fibris 

Ittira  gestorum  famuli  tuorum 

Solve  polluti  labii  reatum 

S  ancte  I  oannes 

The  word  gamut  is  derived  from  gamma,  the  third  letter  of  the 
Greek  alphabet,  because  Guido  d'Arezzo,  who  first  (in  the  eleventh 
century)  represented  notes  by  points  placed  on  parallel  lines, 
denoted  these  lines  by  letters,  and  chose  the  letter  gamma  to  desig- 
nate the  first  line. 

If  we  agree  to  represent  by  I  the  number  of  vibrations  of  the 
fundamental  note  C  or  do  of  the  gamut,  that  is  to  say,  of  the 
deepest  note,  experiment  shows  that  the  numbers  of  vibrations  of 
the  other  notes  of  the  scale  are  those  given  in  this  table  : — 

C  DEFGAB^r 

do          re          mi        fa          sol         la          si          do 

T  9  5  4  3  5.  15  ~ 

1  8  4  3  2  3  8  2 

This  table  does  not  give  the  absolute  numbers  of  the  vibrations 
of  the  various  notes,  but  only  their  relative  numbers.  Knowing 
the  absolute  number  of  vibrations  of  the  fundamental  C,  we  may 
deduce  those  of  the  other  notes  by  multiplying  them  by  |,  Jj,  *,  .  .  . 
or  2  respectively  ;  and  we  thus  find  that  at  the  octave  (174),  the 
number  of  vibrations  is  double  that  of  the  fundamental  note. 

The  scale  may  be  continued  by  taking  the  octaves  of  these  notes 
namely,  c,  d,  e,f,g,  a,  b,  and  again  the  octaves  of  these  last,  and 
so  forth. 

1 74.  Intervals. — An  interval  is  the  ratio  of  one  sound  to  another, 
that  is,  the  relation  between  the  numbers  of  vibrations  which  pro- 
duce these  sounds. 

The  interval  between  two  consecutive  notes  of  the  gamut  is 
called  a  second  \  such  as  the  interval  from  do  to  re,  from  re  to  mi, 
from  mi  to/«,  and  so  on. 

If  between  any  two  notes  which  are  compared,  there  are  one, 
two,  three,  four,  five,  or  six  intermediate  notes,  these  intervals  are 
called  respectively,  a  third,  a.  fourth,  fifth,  sixth,  seventh,  and 
octave.  Thus  the  interval  from  C  to  E  is  a  third,  that  from  C  to 
F  a  fourth,  from  C  to  G  a  fifth,  from  C  to  A  a  sixth,  and  from  C  to 
B  a  seventh,  and  from  C  to  c  an  octave. 


1 68  Acoustics.  [174- 

Although  two  or  more  notes  may  be  separately  musical,  it  does 
not  follow  that,  when  sounded  together,  they  produce  a  pleasant 
sensation.  When  the  ear  can  distinguish  without  fatigue  the  ratio 
between  two  sounds,  which  is  the  case  when  the  ratio  is  simple,  the 
accord  or  co-existence  of  these  two  sounds  forms  a  consonance  ;  but 
if  the  number  of  vibrations  is  in  a  complicated  ratio  the  ear  is  un- 
pleasantly affected  and  we  have  dissonance. 

The  simplest  concord  is  unison,  in  which  the  numbers  of  vibra- 
tions are  equal ;  then  cornes  the  octave,  in  which  the  number 
of  vibrations  of  one  sound  is  double  that  of  the  other ;  then  the 
fifth,  where  the  ratio  of  the  sounds  is  as  3  to  2  ;  the  fourth,  of 
which  the  ratio  is  4  to  3  ;  and  lastly,  the  third,  where  the  ratio  is 
5  to  4. 

If  three  notes  are  sounded  together  they  are  concordant,  when 
the  number  of  their  vibrations  are  as  4  :  5  :  6.  Three  such  notes 
form  a  harmonic  triad,  and  if  sounded  with  a  fourth  note,  which  is 
the  octave  of  the  lowest,  they  constitute  what  is  called  a  major 
chord.  Thus  C,  E,  G,  form  a  major  triad,  G,  B,  d  form  a  major 
triad,  and  F,  A,  c  form  a  major  triad.  C,  G,  and  F  have,  for  this 
reason,  special  names,  being  called  respectively,  the  tonic,  domi- 
nant, and  sub-dominant,  and  the  three  triads  the  tonic,  dominant, 
and  sub-dominant  triads  or  chords  respectively. 

If,  however,  the  ratio  of  any  three  notes  is  as  10  :  12  :  15,  the 
three  sounds  are  slightly  dissonant,  but  not  so  much  as  to  dis- 
qualify them  from  producing  a  pleasant  sensation,  at  least  under 
certain  circumstances.  When  these  three  notes  and  the  octave  to 
the  lower  are  sounded  together  they  constitute  what  in  music  is 
called  a  minor  chord. 

The  intervals  between  the  notes  in  the  scale  are — 

C  to  D  f.  G  to  A  \°. 

D  to  E  \°.  A  to  B  f . 

E  to  F  i-;.  B  to  C  £f. 
F  to  G  f. 

It  will  be  seen  that  there  are  here  three  kinds  of  intervals  ;  the 
interval  f  is  called  a  major  tone,  and  that  of  ^~  a  minor  tone  ;  the 
relation  between  the  major  and  the  minor  tone  is  f  :  ™  =  f£,  and  is 
called  a  comma.  The  interval  ^f  is  called  a  major  semitone.  The 
major  scale  is  formed  of  the  following  succession  of  intervals  :  a 
major  tone,  a  minor  tone,  a  major  semitone,  a  major  tone,  a  minor 
and  a  major  semitone.  It  is  this  succession 


-176]  Musical  Temperament.  169 

which  constitutes  the  scale ;  the  key  note,  or  the  tonic,  may  have 
any  number  of  vibrations  ;  but  once  its  height  is  fixed,  that  of  the 
other  notes  are  always  in  the  above  ratio. 

175.  On  semitones  and  on  scales  with  different  key  notes. — 

It  is  found  convenient  for  the  purpose  of  music  to  introduce  notes 
intermediate  to  the  seven  notes  of  the  gamut  ;  this  is  done  by  in- 
creasing or  diminishing  those  notes  by  an  interval  of  ff,  which  is 
called  a  minor  semitone.  When  a  note  (say  C)  is  increased  by 
this  interval,  it  is  said  to  be  sharpened,  and  is  denoted  by  the 
symbol  Ctf  ,  called  <  C  sharp  ; '  that  is  Ctt  -*-C  =  ff.  When  it  is  de- 
creased by  the  same  interval,  it  is  said  to  be  flattened,  and  is 
represented  thus— B  b ,  called  '  B  flat  ; '  that  is,  B  •+  B  b  =  f  f .  If  the 
effect  of  this  be  examined,  it  will  be  found  that  the  number  of  notes 
in  the  scale  from  C  up  to  c  has  been  increased  from  seven  to 
twenty-one  notes,  all  of  which  can  be  easily  distinguished  by  the 
ear.  Thus,  reckoning  C  to  equal  i,  we  have — 

C        C8        Db        D        Dtf        Eb        E     etc. 

2f.  27  9  75  6  5          „<-„ 

24  25  8  64  5  4          CtC- 

Hitherto  we  have  made  the  note  C  the  tonic  or  key  note.  Any 
other  of  the  twenty-one  distinct  notes  above-mentioned,  for  instance, 
G,  or  F,  or  Ctf  ,  etc,  may  be  made  the  key  note,  and  a  scale  of  notes 
constructed  with  reference  to  it.  This  will  be  found  to  give  rise  in 
each  case  to  a  series  of  notes,  some  of  which  are  identical  with 
those  contained  in  the  series  of  which  C  is  the  key  note,  but  most 
of  them  different.  And  of  course  the  same  would  be  true  for  the 
minor  scale  as  well  as  for  the  major  scale,  and  indeed  for  other 
scales,  which  may  be  constructed  by  means  of  the  fundamental 
triad. 

176.  On  musical  temperament. — The  number  of  notes  that 
arise  from  the  construction  of  the    scales   described   in  the  last 
article  is  enormous  ;   so  much  so  as  to  prove  quite  unmanageable 
in  the  practice  of  music ;  and  particularly  for  music  designed  for 
instruments  with  fixed  notes  such  as  the  pianoforte.      Accordingly 
it  becomes  practically  important  to  reduce  the  number  of  notes, 
which  is  done  by  slightly  altering  their  just  proportions.      This 
process  is  called  temperament.      By  tempering  the  notes,  however 
more  or  less  dissonance  is  introduced,  and  accordingly  several  dif- 
ferent systems  of  temperament  have  been  devised  for  rendering 
this  dissonance  as  slight  as  possible.     The  system  usually  adopted 
is  called  the  system  of  equal  temperament.     It  consists  in  the  sub- 


1 70  Acoustics.  [176- 

stitution  between  C  and  c  of  eleven  notes  at  equal  intervals,  each 
interval  being  the  twelfth  root  of  2,  or  1-05946.  By  this  means  the 
distinction  between  the  semitones  is  abolished,  so  that,  for  example, 
Cff  and  Db  become  the  same  note.  The  scale  of  twelve  notes 
thus  formed  is  called  the  chromatic  scale.  It  of  course  follows  that 
the  major  triad  becomes  slightly  dissonant.  Thus  in  the  diatonic 
scale,  if  we  reckon  C  to  be  i,  E  is  denoted  by  1*25000,  and  G  by 
i  '50000.  On  the  system  of  equal  temperament  if  C  is  denoted  by 
i,  E  is  denoted  by  1-25992  and  G  by  1-49831. 

1/7.  The  number  of  vibrations  producing-  each  note.  The 
tuning  fork. — Hitherto  we  have  not  assigned  any  numerical  value 
to  that  symbol  the  note  C.  In  the  theory  of  music  it  is  common 
to  assign  256  double  vibrations  to  the  middle  C.  This,  however, 
is  arbitrary ;  its  justification  is  the  facility  with  which  this  number 
may  be  subdivided.  An  instrument  is  in  tune  provided  the  in- 
tervals between  the  notes  are  correct,  when  C  is  yielded  by  any 
number  of  vibrations  per  second  not  differing  much  from  256. 
Moreover,  two  instruments  are  in  tune  with  one  another  if,  being 
separately  in  tune,  they  have  any  one  note,  for  instance,  C,  yielded 
by  the  same  number  of  vibrations.  Consequently,  if  two  instru- 
ments have  one  note  (say  C)  in  common,  they  can  then  be  brought 
into  tune  jointly,  by  having  their  remaining  notes  separately  ad- 
justed with  reference  to  that  fundamental  note.  A  tuning  fork  or 
diapason  is  an  instrument  yielding  a  constant  sound,  and  is  used  as 
a'Standard  for  tuning  musical  instruments.  It  consists  of  an  elastic 
steel  rod,  bent  as  represented  in  fig.  149.  It  is  made  to  vibrate 
either  by  drawing  a  bow  across  the  ends,  as  shown  in  the  figure, 
or  by  striking  one  of  the  legs  against  a  hard  body,  or  by  rapidly 
separating  the  two  legs  by  means  of  a  steel  rod.  The  vibration  pro- 
duces a  note  which  is  always  the  same  for  the  same  tuning  fork. 

The  note  is  strengthened  by  fixing  the  tuning  fork  on  a  box 
open  at  one  end  called  a  resonance  box  (178)." 

It  has  been  remarked  for  some  years  that  not  only  has  the  pitch 
of  the  tuning  fork,  that  is,  concert  pitch,  been  getting  higher  in  the 
larger  theatres  of  Europe,  but  also  that  it  is  not  the  same  in  London, 
Paris,  Vienna,  Milan,  etc.  This  is  a  source  of  great  inconvenience 
both  to  composers  and  singers,  and  a  commission  was  appointed 
to  establish  in  France  a  tuning  fork  of  uniform  pitch,  and  to  prepare 
a  standard  which  would  serve  as  an  invariable  type.  In  accord- 
ance with  the  recommendations  of  that  body,  a  normal  timing  fork 


-177] 


Tuning  Fork.     . 


171 


has  been  established,  which  is  compulsory  on  all  musical  esta- 
blishments in  France,  and  a  standard  has  been  deposited  in  the 
Conservatory  of  Music  in  Paris. 

It  makes  870  single  or  435  double  vibrations  in  a  second  and 
yields  the  note  la  of  the  treble  stave  ;  the  do  or  C  of  the  same  stave 
makes  thus  261  double  vibrations  in  a  second. 

The  standard  tuning  fork  adopted  by  the  Society  of  Art  in  Lon- 
don, on  the  recommendation  of  a  committee  of  eminent  musicians, 
makes  264  double  vibrations  in  a  second,  and  gives  the  middle  C 


Fig.  149. 

of  the  treble  stave.  The  corresponding  a  or  la  gives  therefore  440 
vibrations  in  a  second. 

The  middle  C  is  the  note  sounded  by  the  white  key  immediately 
on  the  left  of  the  two  black  keys  which  are  near  the  middle  of  the 
keyboard  of  a  pianoforte.  It  is  designated  in  musical  notation  as 

For  purposes  of  comparison  it  is  convenient  to  call 

this  note  c/9  and  the  next  lower  octave  c  ;  the  octave  lower  than 
this  C,  and  the  still  lower  one  C,,  and  so  on.  The  lowest  note  of 
grand  pianos  is  A,,  which  gives  27-2  vibrations  in  a  second. 

In  like  manner  the  higher  octaves  are  distinguished  by  affixes, 


Acoustics.  [177- 

thus  c"  c'"  riv  and  so  forth.      In  height  the  pianoforte  reaches  to 
air  with  3,520,  or  cv  with  4,224  vibrations  in  a  second. 

The  practical  range  of  musical  sounds  is  comprised  within  40 
and  about  4,000  vibrations  in  a  second  ;  or  a  range  or  7  octaves. 

178.  Resonance  of  air.— The  action  of  the  resonance-box 
in  strengthening  sound  (fig.  149)  may  be  illustrated  by  the  follow 
ing  experiment  (fig.  1 50).  AB  is  a  glass  cylinder  about  8  inches 
in  height,  and  I  to  i^  in  diameter.  If  now  an  ordinary  tuning 
fork  be  made  to  vibrate,  its  sound  is  very  faint,  and  if  it  is  held 


Fig.  150. 

over  the  empty  cylinder  probably  no  alteration  will  be  experienced. 
When,  however,  water  is  slowly  and  noiselessly  poured  into  the 
cylinder,  on  reaching  a  certain  height,  the  previously  faint  sound  is 
far  louder.  Any  other  tuning  fork,  which  yields  a  different  note,  if 
held  over  the  cylinder  will  not  have  its  note  strengthened.  Revert- 
ing now  to  the  original  tuning  fork,  if  while  it  is  still  sounding  and 
its  sound  is  being  strengthened  by  its  nearness  to  the  cylinder,  we 
continue  to  pour  in  water,  the  sound  becomes  as  faint  as  it  was 
originally.  If  now  the  excess  of  water  be  agaki  removed  until 
the  tone  of  the  fork  is  once  more  strengthened,  and  if,  removing  the 


-179]  Harmonics.  173 

fork,  we  sound  the  column  again  by  blowing  into  it,  we  find  that 
the  column  of  air  emits  the  same  note  as  the  tuning  fork.  Hence 
then  the  tuning  fork  could  set  a  column  of  air  of  a  particular  length 
in  vibration  so  as  to  produce  the  same  note ;  and  this  adding  itself 
to  the  original  note  strengthened  it. 

179.  Compound  musical  tones.  Harmonics.  Overtones. — 
We  have  already  seen  that  there  is  a  peculiar  quality  or  timbre,  as 
it  is  called,  by  which  the  notes  of  different  instruments  are  cha- 
racterised. Thus  we  readily  distinguish  between  the  note  C  when 
sounded  on  a  pianoforte  and  the  same  note  sounded  on  an  organ 
or  a  trumpet.  This  peculiarity  of  the  tone  is  due  to  the  fact  that 
only  in  very  few  cases  does  an  instrument  give  a  pure  note  but 
that  in  most  cases  it  is  accompanied  by  a  series  of  upper  notes  or 
harmonics.  To  understand  what  these  are  we  may  refer  to  191, 
in  which  it  is  stated  that  by  successively  intensifying  the  current 
of  air,  we  get  in  a  stopped  pipe  a  succession  of  notes  the  numbers 
of  whose  vibrations  are  as  the  series  of  odd  numbers,  i,  3,  5,  7,  etc. 
So  too  if  we  similarly  sound  an  open  pipe  we  get  the  series  of  notes 
whose  numbers  of  vibrations  are  represented  by  the  series  of  even 
numbers,  i,  2,  3,  4,  5,  etc.  These  are  called*  respectively  the  odd 
and  even  harmonics  of  the  primary  note. 

Now  if  we  sound  a  particular  note  on  the  piano,  by  a  little  at- 
tention a  practised  ear  can  discover  that  the  primary  note  is  ac- 
companied by  a  series  of  higher  notes  each  of  which  gradually 
gets  fainter.  These  upper  notes  may  be  detected,  and  the  com- 
pound nature  of  the  primary  sound  analysed  even  by  an  unpractised 
ear  by  the  use  of  resonance  'globes  which  Helmholtz  devised  for 
this  purpose.  These  instruments,  one  of  which  is  represented  in 
fig.  151,  are  an  application  of  the  principle  explained  in  the  fore- 
going paragraph.  They  are 
small  hollow  spheres,  the  pro- 
jection ^,  which  has  a  small 
hole,  is  placed  in  the  ear  while 
the  wider  aperture  a  is  directed 
towards  the  source  of  sound. 
Each  of  these  resonators  is 
constructed  or  tuned  for  a  par- 
ticular note  ;  so  that  if  having 
sounded  the  string  of  a  piano-  Flg>  ISI> 

forte,  we  hold  near  it  a  resonator  tuned  for  a  particular  note,  this 
note  if  present  will  be  intensified.  Thus,  if  we  depress  the  key  c. 


Acoustics.  [179- 

we  hear  no  particular  strengthening  if  a  resonator  sounded  for  g 
be  held  near  the  ear  ;  but  when  the  resonators  sounded  for  c/t  glt 
c//y  are  used  we  hear  them  powerfully  respond  when  held  to  the  ear. 
Hence  the  notes  cfl  gn  c,,,  are  contained  in  the  mass  of  sound  which 
is  produced  when  the  key  ct  is  depressed. 

Helmholtz's  researches  show  that  the  different  timbre  or 
quality  of  the  sounds  yielded  by  different  instruments  is  due  to  the 
fact  that  they  are  accompanied  in  each  case  by  special  har- 
monics or  overtones  in  varying  intensity. 

Helmholtz's  principal  results  are  as  follows  : — 

Simple  tones — those,  that  is  to  say,  without  any  admixture  of 
overtones — are  most  easily  produced  when  a  tuning  fork  is  held 
near  a  resonance-box  of  suitable  length.  These  notes  are  soft  and 
are  free  from  all  sharpness  and  roughness. 

Fhite  notes  are  also  nearly  pure,  for  their  overtones  are  very 
feeble.  Wide-stopped  organ  pipes  give  the  fundamental  note 
almost  perfectly  pure,  narrower  ones  give  along  with  it  the  fifth  of 
the  octave. 

Wide  open  pipes  give  the  octave  along  with  the  fundamental 
note  ;  and  narrow  ones  give  a  series  of  overtones. 

The  overtones  present  in  the  sound  of  stretched  strings  depend 
on  their  substance  and  on  the  manner  in  which  they  are  made  to 
sound.  In  good  pianos  the  overtones  are  powerful  up  to  the  sixth. 
In  stringed  instalments  the  fundamental  note  is  comparatively 
stronger  than  in  pianos  ;  the  first  overtones  are  feebler,  the  higher, 
from  the  sixth  to  the  tenth,  on  the  contrary,  are  far  more  distinct, 
and  produce  the  penetrating  character  of  the  sound  of  stringed 
instruments. 

Metallic  rods  and  plates  produce,  along  with  the  fundamental 
note,  a  series  of  very  high  overtones  which  are  discordant  with 
each  other,  but  are  continuous  and  of  equal  strength  with  the 
primary  note.  Thus  is  produced  that  peculiarity  known  as  a 
metallic  sound. 

.,  By  the  occurrence  of  the  lower  harmonics  along  with  the  primary 
note  the  tone  is  more  sonorous,  richer  and  deeper  than  the  primary 
note ;  by  the  occurrence  of  the  higher  overtones,  the  clang  acquires 
its  penetrating  character. 


-181]  Transverse  Vibrations  of  Strings.  175 


CHAPTER   III. 

TRANSVERSE  VIBRATIONS  OF  STRINGS.     STRINGED  INSTRUMENTS. 

1 80.  Transverse  vibrations   of  strings. — We  have  already 
seen  (156),  that  when  an  elastic  string,  stretched  at  the  ends,  is 
removed  from  its  position  of  equilibrium,  it  reverts  to  it  as  soon  as 
it  is  let  go,  making  a-  series  of  vibrations  which  produce  a  sound. 
The  strings  used  in  music  are  commonly  of  catgut  or  metallic  wire. 
The  vibrations  which  strings  experience  may  be  either  transversal 
or  longitudinal,  but  practically  the   former  are  alone  important. 
Tansversal  vibrations  may  be  produced  by  drawing  a  bow  across, 
the  string,  as  in  the  case  of  the  violin  ;  or  by  striking  the  string, 
as  in  the  case  of  the  pianoforte  ;  or  by  pulling  them  transversely 
and  then  letting  them  go  suddenly,  as  in  the  case  of  the  guitar^  and 
the  harp. 

181.  Laws    of   the   transverse  vibrations   of  strings. — The 
number   of  transverse  vibrations  which  a   string   can   give   in   a 
certain  time,  that  is,  the  sound  it  yields,  vary  with  its  length,  its 
diameter,  its  tension,  and  with  its  specific  gravity  in  the  following 
manner  : 

The  tension  being  constant,  the  number  of  vibrations  in  a  second 
is  inversely  as  the  length  ;  that  is,  that  if  a  string  makes  18  vibra- 
tions in  a  second  for  instance,  it  will  make  36  if  its  length  is  halved, 
54  if  its  length  is  one-third,  and  so  on.  On  this  property  depends 
the  violin,  the  centre  basso,  etc.,  for  in  these  instruments  by 
pressing  the  string  with  a  finger,  the  length  is  reduced  or  increased 
at  pleasure,  and  the  number  of  vibrations,  and  therewith  the  note 
is  regulated. 

With  strings  of  the  same  length  and  tension  the  number  of 
vibrations  in  a  second  is  inversely  as  the  diameter  of  the  string ;  that 
is,  the  thinner  a  string,  the  greater  its  number  of  vibrations,  and 
the  higher  its  pitch.  In  the  violin,  the  treble  string,  which  is  the 
thinnest,  makes  double  the  number  of  vibrations  of  that  which 
would  be  made  by  a  string  twice  its  size,  that  is  to  say,  the  dia- 
meter of  which  is  twice  as  great. 


ij6  A  coustics.  [181- 

The  number  of  vibrations  in  a  second  is  directly  as  the  square 
root  of  the  stretching  weight  or  tension  ;  that  is,  that  when  the 
tension  of  a  string  is  four  times  as  great,  the  number  of  vibrations 
is  doubled  ;  when  the  tension  is  nine  times  as  great,  the  number  is 
trebled,  and  so  on.  This,  then,  furnishes  a  means  of  altering  the 
character  of  a  note  by  stretching,  as  is  done  in  stringed  instru- 
ments. 

Other  things  being  equal,  the  number  of  vibrations  in  a  second 
of  a  string  is  inversely  as  the  square  root  of  its  density.  Hence, 
the  greater  the  density  of  the  materials  of  which  strings  are  made, 
the  less  easily  they  vibrate,  and  the  deeper  are  the  sounds  they 
yield. 

From  the  preceding  laws  it  will  be  seen  how  easy  it  is  to  vary 
the  number  of  the  vibrations  of  strings  and  make  them  yield  an 
extreme  variety  of  sounds,  from  the  deepest,  to  the  highest  used  in 
music. 

182.  Verification  of  the  laws  of  the  vibrations  of  strings. 
Sonometer. — This  may  be  effected  by  means  of  an  instrument 
called  the  sonometer  or  monochord.  It  consists  of  a  thin  wooden 
box  to  strengthen  the  sound.  On  this  there  are  two  fixed  bridges 
A  and  B  (fig.  152),  over  which  pass  the  strings  AB,  CD,  which 


Fig.   152. 

are  commonly  metal  wires.  These  are  fastened  at  one  end,  and 
stretched  at  the  other  by  a  weight  P,  which  can  be  increased  at 
will.  By  means  of  a  third  movable  bridge  D,  the  length  of  that 
portion  of  the  wire  which  is  to  be  put  in  vibration  can  be  altered 
at  pleasure 

If  two  strings  are  taken,  which  are  identical  in  all  respects,  and 
are  stretched  by  equal  weights,  they  will  be  found,  on  being  struck, 
to  yield  the  same  sound.  If  now  one  of  them  be  divided  by  the 


-183]  Stringed  Instruments.  177 

movable  bridge  D  into  two  equal  parts,  the  sound  yielded  by  CD 
will  be  the  higher  octave  of  that  yielded  by  the  entire  string  AB, 
which  shows  that  the  number  of  vibrations  is  doubled,  and  thus 
verifies  the  law. 

To  verify  the  second  law,  the  bridge  D  is  removed.  If  the  string 
AB  is  taken  so  that  it  has  double  the  diameter  of  the  other,  but  both 
stretched  by  the  same  weight,  it  will  be  found  that  the  sound  which 
the  thinnest  string  yields  is  the  next  higher  octave  of  that  yielded 
by  AB  ;  proving  thus  that  the  number  of  vibrations  is  doubled. 

The  two  strings  being  of  the  same  diameter,  and  the  same 
length,  if  the  weight  which  stretches  the  one  be  four  times  that 
which  stretches  the  other,  the  sound  yielded  by  the  first  is  the 
higher  octave  of  that  of  the  second,  which  shows  that,  the  number 
of  vibrations  is  doubled  ;  when  the  weight  is  nine  times  as  great, 
the  sound  is  the  higher  octave  of  the  fifth  of  the  former. 

The  fourth  law  is  established  by  using  strings  of  different  den- 
sities, but  of  the  same  dimensions,  and  stretched  to  the  same 
extent. 

183.  Stringed  instruments. — Stringed  musical  instruments 
depend  on  the  production  of  transverse  vibrations.  In  some,  such 
as  the  piano,  the  sounds  are  constant,  and  each  note  requires  a 
separate  string  :  in  others,  such  as  the  violin  and  guitar,  the 
sounds  are  varied  by  the  fingering,  and  can  be  produced  by  fewer 
strings. 

In  the  piano  the  vibrations  of  the  strings  are  produced  by  the 
stroke  of  the  hammer,  which  is  moved  by  a  series  of  bent  levers 
communicating  with  the  keys.  The  sound  is  strengthened  by  the 
vibrations  of  the  air  in  the  soimding  board 'on  which  the  strings  are 
stretched.  Whenever  a  key  is  struck,  a  damper  is  raised,  which 
falls  when  the  finger  is  removed  from  the  key  and  stops  the  vibra- 
tions of  the  corresponding  string.  By  means  of  a  pedal  all  the 
dampers  can  be  simultaneously  raised,  and  the  vibrations  then 
last  for  some  time. 

The  harp  is  a  sort  of  transition  from  the  instruments  with  con- 
stant to  those  with  variable  sounds.  Its  strings  correspond  to  the* 
natural  notes  of  the  scale  ;  by  means  of  the  pedals  the  lengths  of 
the  vibrating  parts  can  be  changed,  so  as  to  produce  sharps  and 
flats.  The  sound  is  strengthened  by  the  sounding  box,  and  by 
the  vibrations  of  all  the  strings  harmonic  with  those  played. 

In  the  violin  and  guitar  each  string  can  give  a  great  number  of 
sounds  according  to  the  length  of  the  vibrating  part,  which  is  de- 

N 


Acoustics. 


[183- 


termined  by  the  pressure  of  the  fingers  of  the  left  hand  while  the 
right  hand  plays  the  bow,  or  the  strings  themselves.  In  both  these 
instruments  the  vibrations  are  communicated  to  the  upper  face  of 
the  sounding  box,  by  means  of  the  bridge  over  which  the  strings 
pass.  These  vibrations  are  communicated  from  the  upper  to  the 
lower  face  of  the  box,  either  by  the  sides,  or  by  an  intermediate 
piece  called  the  sound  post.  The  air  in  the  interior  is  set  in  vibra- 
tion by  both  faces,  and  the  strengthening  of  the  sound  is  produced 
-by  all  these  simultaneous  vibrations.  The  value  of  the  instrument 
consists  in  the  perfection  with  which  all  possible  sounds  are  in- 
tensified, which  depends  essentially  on  the  quality  of  the  wood, 
and  the  relative  arrangement  of  the  parts. 

Instruments  of  the  class 
of  the  violin  are  very  difficult 
to  play,  and  require  a  very 
delicate  ear  ;  but  in  the  hands 
of  skilful  artists,  they  produce 
marvellous  effects.  They  are 
the  very  soul  of  an  orchestra, 
and  the  most  beautiful  pieces 
of  music  have  been  composed 
for  them. 

184.  Longitudinal  vibra- 
tions of  string's  and  rods. — 
When  a  violin  bow  is  passed 
over  the  string  of  the  mono- 
chord  at  a  very  acute  angle, 
an  unpleasant  but  powerful 
tone  is  heard.  If  the  tension 
of  the  string  be  altered,  there 
is  no  change  in  the  note.  If 
the  string  be  touched  in  the 
middle  it  yields  the  octave 
when  the  bow  is  passed  over 
it.  These  tones  are  pro- 
duced by  longitudinal  vibra- 
tions, their  pitch  varies  in- 
versely as  the  length  of  the 
string,  but  is  independent  of 
the  thickness  and  tension.  In  like  manner  if  a  glass  tube  be 
grasped  in  the  middle,  and  rubbed  lengthwise  with  a  wet  cloth,  a 


Fig.  153- 


- 185]  Production  of  Sound  in  Pipes.  1 79 

penetrating  but  not  unpleasant  tone  is  produced.  If  grasped  at  a 
quarter  of  its  length  and  if  the  shorter  part  be  made  to,  vibrate,  the 
octave  of  the  former  tone  is  obtained. 

Marloye's  harpy  tig.  153,  is  an  arrangement  which  illustrates  the 
sounds  produced  by  the  longitudinal  vibration  of  rods.  It  consists 
of  a  series  of  deal  rods  of  different  lengths  and  thicknesses.  They 
are  sounded  by  rubbing  the  rods  lengthwise  with  resined  fingers.  A 
series  of  notes  of  varying  pitch  is  thus  produced,  which  by  a  skilful 
artist  is  far  from  unpleasing. 

The  tuning-fork,  the  triangle,  and  musical  boxes  are  examples 
of  the  transverse  vibration  of  rods.  In  musical  boxes  small  plates 
of  steel  of  different  dimensions  are  fixed  on  a  rod,  like  the  teeth  of 
a  comb.  A  cylinder  whose  axis  is  parallel  to  this  rod,  and  whose 
surface  is  studded  with  steel  teeth,  arranged  in  a  certain  order,  is 
placed  near  the  plates.  By  means  of  a  clockwork  motion  the 
cylinder  rotates,  and  the  teeth  striking  the  steel  plates  set  them  in 
vibration,  producing  a  tune,  which  depends  on  the  arrangement  of 
the  teeth  on  the  cylinder. 


CHAPTER    IV. 

SOUND   PIPES  AND   WIND    INSTRUMENTS. 

185.  Production  of  sound  in  pipes. — Sound  pipes  are  hollow 
pipes  or  tubes  in  which  sounds  are  produced  by  making  the. en- 
closed column  of  air  vibrate.  In  the  cases  hitherto  considered 
the  sound  results  from  the  vibrations  of  solid  bodies,  and  the  air 
only  serves  as  a  vehicle  for  transmitting  them.  In  wind  instru- 
ments, on  the  contrary,  when  the  sides  of  the  tube  are  of  adequate 
thickness,  the  enclosed  column  of  air  is  the  sounding  body.  In 
fact,  the  substance  of  the  tubes  is  without  influence  on  the  primary 
tone  ;  with  equal  dimensions  it  is  the  same  whether  the  tubes  are 
of  glass,  of  wood,  or  of  metal.  These  different  materials  simply  do 
no  more  than  give  rise  to  different  harmonics,  and  impart  a  dif- 
ferent timbre  to  the  compound  tone  produced. 

If  tubes  were  simply  blown  into,  there  could  be  no  sound  ;  there 
would  merely  be  a  continuous  progressive  motion  of  the  air.  To 
produce  a  sound,  by  some  means  or  other  a  rapid  succession  of 

N  2 


i8o 


Acoustics. 


[185- 


condensations  and  rarefactions  must  be  produced,  which  are  then 
transmitted  to  the  whole  column  of  air  in  the  tube.  Hence  the  ne- 
cessity of  having  a  mouthpiece,  that  is,  the  end  by  which  air  enters, 
so  shaped  that  the  air  enters  in  an  intermittent,  and  not  a  contin- 
uous manner.  From  the  arrangement  made  use  of  to  set  the 
enclosed  air  in  vibration,  wind  instruments  are  divided  into  mouth 
instruments  and  reed  instruments. 

1 86.  Mouth  instruments. — In  mouth  instruments  all  parts  of 
the  mouthpiece  are  fixed.     The  pipes  are  either  of  wood  or  metal, 


N 


Fig.   154.  Fig.  155.  Fig.  156.      Fig.  157. 


Fig.  158. 


rectangular  or  cylindrical,  and  are  always  long  as  compared  with 
the  diameter.  Fig.  154  represents  a  wooden  rectangular  organ 
pipe;  fig.  155  gives  a  longitudinal  section  by  which  the  internal 
details  are  seen.  The  lower  part  P,  by  which  air  enters,  is  called 
the_/£>0/;  it  emerges  through  a  narrow  slit  i,  and,  on  the  opposite 


-187]  Reed  Instruments.  181 

side,  is  a  transverse  aperture  called  the  mouth  ;  a  and  b  are  the 
lips,  the  upper  one  of  which  is  bevelled. 

The  current  of  air  arriving  by  the  mouth,  strikes  against  the 
upper  lip,  is  compressed,  and  by  its  elasticity  reacts  upon  the 
current  and  stops  it.  This,  however,  only  lasts  for  an  instant,  for, 
as  the  air  escapes  at  ab,  the  current  from  the  foot  continues,  and  so 
on  for  the  whole  time. 

In  this  way,  pulsations  are  produced,  which,  transmitted  to  the 
air  in  the  pipe,  make  it  vibrate,  and  a  sound  is  the  result.  In  order 
that  a  pure  note  may  be  produced,  there  must  be  a  certain  relation 
between  the  form  of  the  lips  and  the  magnitude  of  the  mouth  ;  the 
tube  also  ought  to  have  a  great  length  in  comparison  with  its 
diameter.  The  number  of  vibrations  depends  in  general  on  the 
dimensions  of  the  pipe  and  the  velocity  of  the  current  of  air. 

The  mouthpiece  we  have  described  is  used  in  organs.  Fig.  156 
represents  another  modification  much  in  use  in  organ  playing,  and 
fig.  1 57  gives  a  longitudinal  section.  The  letters  indicate  the  same 
parts  as  in  fig.  155.  Fig.  158  shows  the  mouthpiece  of  a  flageolet 
and  whistle.  In  the  German  flute  the  mouthpiece  consists  of  a 
small  lateral  circular  aperture  in  the  pipe.  By  means  of  his  lips 
the  player  causes  the  current  of  air  to  graze  against  the  edge  of  this 
aperture. 

187.  Reed  instruments. —  In  reed  instruments  the  air  is  set 
in  vibration  by  means  of  elastic  tongues  or  plates,  which  are 
called  reeds,  and  which  are  divided  into  free  reeds  and  beating 
reeds. 

Beating  reed.  This  consists  of  a  piece  of  wood  or  metal,  a  (fig. 
1 60),  which  is  grooved  like  a  spoon.  It  is  fixed  to  a  kind  of 
stopper,  K,  perforated  by  a  hole,  which  connects  the  cavity  with 
a  long  pipe,  T.  The  groove  is  covered  by  a  brass  plate,  /,  which 
is  called  the  tongue.  In  its  ordinary  position  this  is  slightly  away 
from  the  edges  of  the  groove,  but  being  very  flexible,  readily  ap- 
proaches, and  closes  it.  Lastly,  a  cuived  wire  br}  presses  against 
the  tongue,  and  can  be  moved  up  and  down. 

The  vibrating  part  of  the  tongue  can  thereby  be  shortened  or 
lengthened  at  will,  and  the  number  of  vibrations  thus  regulated. 
By  means  of  this  wire,  reed  pipes  are  tuned. 

The  reed  is  fitted  to  the  top  of  a  rectangular  pipe  KN,  called 
the  wind  channel.  This  is  closed  everywhere,  except  at  the 
bottom,  where  it  can  be  fitted  on  a  bellows.  In  models  of  reed 
pipes  used  in  illustrating  lectures,  the  sides  of  the  upper  part  of 


182 


Acoustics. 


[187- 


the  tube  are  made  of  glass,  so  as  to  show  the  construction  of  the 
reed.     This  arrangement  is  represented  in  fig,  159. 

When  air  arrives  in  the  wind  channel,  it  first  passes  between 
the  tongue  and  the  groove,  and  escapes  by  the  pipe  T  ;  but  as  the 
velocity  increases,  the  tongue  strikes  against  the  edge  of  the 
groove,  and  closing  it  completely,  the  current  is  stopped.  But,  in 
virtue  of  its  elasticity,  the  tongue  reverts  to  its  original  position, 


Fig.  159- 


Fig.  1 60. 


Fig.   161. 


and  thus  by  a  series  of  alternate  openings  and  closings,  the  same 
series  of  pulsations  are  produced  as  in  mouth  instruments  ;  hence 
is  formed  a  sound  which  is  higher  the  more  rapid  the  current  of 
air. 

Free  reed.  Grenie  invented  in  1810  a  kind  of  reed  called  a.  free 
reed,  because  the  tongue,  instead  of  striking  against  the  edges  of 
the  groove,  like  the  reed  described  above,  grazes  them  so  as  to  os- 
cillate backwards  and  forwards.  The  groove  consists,  in  this  case, 


-189]  Nodes  and  Loops.  183 

of  a  small  wooden  box,  ac,  fig.  161,  the  front  of  which  is  of  brass 
plate.  In  the  middle  of  this  is  a  longitudinal  slit,  in  which  is  ap- 
plied the  tongue,  which  can  oscillate  freely  backwards  and  forwards 
so  as  to  allow  air  to  pass,  which  it  closes  each  time  it  grazes  the 
edges  of  the  slit.  In  this  case  also  a  wire,  r,  regulates  the  length 
of  the  vibrating  part  of  the  tongue. 

A  reed  can  be  very  simply  made  from  a  piece  of  straw.     About 
an  inch  from  a  knot  an  incision  is  made  at  r,  (fig.  162),  with  a 


Fig.   162. 

sharp  penknife,  which  is  about  a  quarter  as  deep  as  the  diameter 
of  the  straw  ;  and  then  by  laying  the  knife  flat  the  straw- is  slit  as 
far  as  the  knot ;  the  strip  r  r,  thus  produced  forms  a  reed  joined 
with  the  pipe  .y  r.  The  note  of  this  pipe  depends  on  the  length  of 
the  tube  s  r  and  is  higher  the  shorter  the  tube  is  made.  In  order 
to  sound  the  pipe,  the  whole  length  of  the  reed  is  placed  in  the 
mouth  and  the  lips  firmly  closed. 

1 88.  Bellows. — In  acoustics  a  bellows  is  an  apparatus  by  which 
wind  instruments,  such  as  the  syren  and  organ  pipes,  are  worked. 
Between  the  four  legs  of  a  table  there  is  a  pair  of  bellows,  S  (fig. 
163),  which  is  worked  by  means  of  a  pedal,  P.     D  is  a  reservoir  of 
flexible  leather,  in  which  is  stored  the  air  forced  in  by  the  bellows. 
If  this  reservoir  is  pressed  by  means  of  weights  on  a  rod,  T,  moved 
by  the  hand,  the  air  is  driven  through  a  pipe,  A,  into  a  wind  chest, 
mn,  fixed  on  the  table.  '  In  this  chest  there  are  small  holes  closed 
by  leather  valves  s  (fig.  164).     These  can  be  opened  by  pressing 
on  keys,  a,  in  front  of  the  box.     Below  the  valve  is  a  spring,  r, 
which  raises  the  valve  when  the  key  is  not  depressed.     The  sound 
pipe  is  placed  in  one  of  these  holes. 

189.  Nodes  and  loops. — Experiment  shows,  that  when  a  pipe 
is  sounded,  the  column  of  air  is  subdivided  into  equal  parts,  vibrat- 
ing in  unison,  and  separated  by  surfaces  where  the  velocity  of  air 
is  null.     These  fixed  parts  are  called  nodes  :  and  the  parts  between 
the  nodes  where  the  column  of  air  is  in  a  state  of  vibration  is  called 
a  loop,  or  a  ventral  segment. 

It  will  be  seen  afterwards,  that  one  and  the  same  pipe  may  be 
made  to  yield  several  sounds,  and  that  the  nodes  and  ventral  seg- 
ments then  alter  their  position.  When  a  pipe  closed  at  one  end,  a 


1 84 


Acoustics. 


[189- 


stopped  pipe,  is  made  to  yield  its  fundamental  sound,  that  is,  the 
deepest  one,  the  bottom  is  always  a  node,  and  the  mouthpieces  a 
ventral  segment.  An  open  pipe  when  sounded  has  a  ventral 
segment  at  each  end  ;  and  if  it  yields  the  fundamental  sound,  there 
is  a  single  node  in  the  middle. 


Fig.   163. 

When  an  aperture  is  opened  in  the  side  of  a  sounding  pipe,  the 
sound  does  not  change  if  the  aperture  corresponds  to  a  loop  ;  but 
if  it  corresponds  to  a  node,  the  sound  .is  altered,  for  this  node  then 
becomes  a  loop.  This  property  is  used  in  wind  instruments  like 
the  flute,  the  clarionet,  along  which  holes  are  made  which  can  be 
closed  by  the  fingers,  or  by  the  aid  of  keys. 

The  formation  of  nodes  and  loops  is  far  from  being  restricted 
to  sounding  tubes.  Strings,  plates  and  membranes,  when  they 
vibrate,  exhibit  parts  which  are  fixed,  and  parts  which  are  very 
mobile,  that  is  to  say,  nodes  and  loops.  , 


-191] 


Pitch  Pipes. 


185 


190.  Laws  of  the  v4bration  of  air  in  pipes. — The  vibrations 
of  air  in  pipes  present  two  cases  -according  as  they  are  open  or 
stopped. 

Laws  of  stopped  pipes.  When  having  placed  a  stopped  pipe  on 
the  bellows,  air  is  slowly  passed,  the  deepest  note,  the  fundamental, 
sound,  is  produced.  If,  then,  we  denote  by  I  the  corresponding 
number  of  vibrations,  when  the  current  of  air  is  forced,  we  suddenly 
get  the  sound  corresponding  to  3  ;  and  if  the  wind  be  still  more 
forced,  we  have  successively  the  sounds  5,  7,  etc.  ;  that  is  to  say, 
sounds  which  by  their  pitch  correspond  to  vibrations  3,  5,  7,  etc. 
times  as  numerous  as  those  of  the  fundamental  sound.  Hence 
closed  pipes,  when  the  air  is  forced,  give  successively  sounds  re- 
presented by  the  series  of  odd  numbers. 

The  sounds  3,  5,  7,  etc.,  are  called  the  harmonics  of 
the  fundamental  note  I. 

2.      With  pipes  of  different  lengths,  the  number  of 
vibrations  corresponding  to  the  fundamental  note  are  in-      / 
versely  as  the  lengths  ;  that  is  to  say,  that  a  pipe,  which      all 
is  half  as  long  as  another,  will  yield  a  sound  which  is  the 
octave  of  that  yielded  by  this  pipe. 

Laws  of  open  pipes.  The  fundamental  note  being 
still  represented  by  unity,  the  harmonics  obtained  by 
forcing  the  wind  are  successively  represented  by  2,  3,  4, 
5,  6,  etc.,  that  is,  by  the  natural  series  of  numbers. 

The  fundamental  note  of  an  open  pipe  is  always  an 
octave  higher  than  the  fundamental  note  of  a  closed  pipe 
of  the  same  length. 

These  laws  are  known  as  Bernoulli's  laws  from  the 
name  of  their  discoverer,  Daniel  Bernoulli. 

191.  Pitch  pipe. — Instead  of   organ  pipes  of  various 
lengths   and  bellows,   these   laws  may  be   conveniently 
demonstrated   by  means  of  a  pitch  pipe,  hg.  165,. which 
is  a  small  sound  pipe  with  a  movable  graduated  stopper. 
If  having  closed  the  pipe  at  its  full  length,  we  blow  into 
it,  we  get  the  fundamental  note  of  the  stopped  pipe,  say 

c  ;  if  now  we  blow  into  it  more  strongly  we  get  the  note    Fig.  165. 
gt,  which  is  the  major  fifth  of  the  higher  octave  of  c,  and 
more  strongly  still  the  note   e/n  which  is  the   major  third  of  its 
second  octave,  and  so  on  for  the  others. 

In  like  manner  having  just  closed  the  pipe,  if  we  push  in  the 
stopper  until  its  length  is  one  half  and  sound  it,  we  get  the  higher 


1 86 


A  caustics. 


[191 


octave  of  the  fundamental  note,  by  making  it  ^  its  original  length 
we  get  the  major  third  of  c,  and  so  for  any  other  aliquot  part. 

By  removing  the  stopper  altogether  we  have  an  open  pipe,  and 
the  note  c  which  it  yields  is  the  octave  of  the  stopped  one,  and 
.this  sounded  by  increasingly  powerful  currents  of  air  gives  the 
following  series  of  notes,  c,  c^  gtl  ciu  etjl  and  so  forth. 

192.  Wind  Instruments. — Wind  instruments  are  straight  or 
curved  tubes,  which  are  sounded  by  means  of  a  current  of  air  forced 
into  them.  They  have  all  an  aperture  by  which  air  is  forced  into 
them,  and,  according  to  the  form  of  this  aperture,  they  are  divided 
into  mouth  instruments  and  reed  instruments  ;  in  some,  such  as 
the  organ,  the  notes  are  fixed,  and  require  a  separate  pipe  for  each 
note  ;  in  others  the  notes  are  variable,  and  are  produced  by  only 
one  tube  ;  the  flute,  horn,  etc.,  are  of  this  class. 


Fig.    1 66. 


The  Pandaean  pipe,  the  flageolet,  and  the  German  flute  are  mouth 
instruments.  The  principal  reed  instruments  are  the  clarionet,  the 
oboe,  the  cornopean,  and  the  bassoon. 

The  Pandtzan  pipe,  fig.  166,  consists  of  tubes  of  different  sizes 
corresponding  to  the  different  notes  of  the  gamut. 

In  the  organ  the  pipes  are  of  various  kinds,  namely,  mouth  pipes, 
open  and  stopped,  and  reed  pipes  with  apertures  of  various  shapes. 
The  air  is  furnished  by  means  of  bellows,  from  which  it  passes  into 
the  wind  chest,  and  thence  into  any  pipe  which  is  desired  ;  this  is 
effected  by  means  of  valves  which  are  opened  by  depressing  keys 
like  those  of  the  piano.  In  the  larger  and  richer  organs  there  are 
several  rows  of  key-boards  arranged  at  different  heights. 

In  thejtfttte,  the  mouthpiece  consists  of  a  simple  lateral  circular 
aperture  ;  the  current  of  air  is  directed  by  means  of  the  lips,  so 
that  it  grazes  the  edge  of  the  aperture.  The  holes  at  different 
distances  are  closed  either  by  the  fingers  or  by  keys  ;  when  one  of 


-193] 


Human  Voice. 


87 


the  holes  is  opened,  a  loop  is  produced  in  the  corresponding  layer 
of  air,  which  modifies  the  distribution  of  nodes  and  loops  in  the 
interior,  and  thus  alters  the  note.  The  whistling  of  a  key  is 
similarly  produced. 

Mouth  instruments.  In  the  trumpet,  the  horn,  the  trombone, 
cornet-a-piston,  and  ophicleide,  the  lips  form  the  reed,  and  vibrate 
in  the  mouthpiece  (fig.  167),  which  terminates  in  a 
smaller  tube  by  which  it  can  be  affixed  to  the 
instrument.  In  the  horn,  different  tones  are  pro- 
duced by  altering  the  distance  of  the  lips.  In  the 
trombone,  one  part  of  the  tube  slides  within  the  other, 
and  the  performer  can  alter  at  will  the  length  of  the 
tube,  and  thus  produce  higher  or  lower  notes.  In 
the  cornet-a-piston,  the  tube  forms  several  convo- 
lutions ;  pistons  placed  at  different  distances  can, 
when  played,  cut  off  communications  writh  other 
parts  of  the  tube,  and  thus  alter  the  length  of  the 
vibrating  column  of  air. 

193.  The  Human  Voice.— If  we  bevel  off  the-  Fig.  167. 
ends  of  a  piece  of  gutta-percha  or  of  wooden  tubing,  so  that  two 
summits  are  left,  and  if  now  two  pieces  of  thin  vulcanised  india- 
rubber  or  leather  be  stretched  and  tied  between  them  so  as  to 
leave  a  narrow  slit,  we  have  then  a  sort  of  membranous  reed  pipe 
(fig.  1 68).  For  if  we  blow  into  the  tube  we  get  a  note  which  is 
higher  the  tighter  the  lips  are  stretched; 
and  the  vibrations  of  the  lips  which 
form  the  slit  can  be  distinctly  seen. 

This  simple  experiment  well  illus- 
trates the  manner  in  which  the  sound 
of  the  human  voice  is  produced  in  the 
glottis  at  the  top  of  the  windpipe.  The 
windpipe  becomes  narrow  towards  the 
top,  ends  in  a  slit,  formed  by  elastic 
bands,  the  vocal  chords. 

These  are  joined  back  and  front 
to  the  material  of  the  windpipe,  and  can  be  more  or  less  tightly 
stretched  by  means  of  various  muscles.  The  human  voice 
may  thus  be  regarded  as  a  reed  pipe  formed  by  two  elastic 
bands. 

The  essential  sonorous  part  of  the  human  voice  is  formed  by  the 
vowels.      They  acquire  their  special  sound  by  the  fact  that  to  pro- 


Fig.  168. 


1 88  Acoustics.  [193- 

duce  them  in  each  case,  the  cavity  of  the  mouth  spontaneously 
alters  its  shape  and  thus  acts  as  a  special  resonator  to  each  sound. 
The  consonants  are  noises  which  formed  by  the  lips,  tongue,  and 
teeth  accompany  the  vowels  at  their  commencement  and  cessation. 
The  sounds  by  which  the  consonants  are  produced  are  much 
less  intense  than  the  vowel  sounds.  Hence  they  are  inaudible  at 
distances  at  which  the  vowel  sounds  can  be  distinctly  heard. 
Therefore,  in  speaking  with 'people  hard  of  hearing,  it  is  by  no 
means  necessary  to  speak  louder,  but  it  is  sufficient  to  intensify  the 
consonants.  Indeed,  distinctness  of  speech  does  not  depend  on 
loud  screaming,  but  is  produced  by  careful  articulation. 


-194]  Heat.  189 


BOOK   V. 

HEAT. 


CHAPTER    I. 
GENERAL   EFFECTS   OF   HEAT.      THERMOMETERS. 

194.  Keat.  Hypothesis  as  to  its  nature.— The  sensations  of 
heat  and  cold  are  familiar  to  all  of  us.  In  ordinary  language  the 
term  heat  is  not  only  used  to  express  a  particular  sensation,  but 
also  to  describe  that  particular  state  or  condition  of  matter  which 
produces  this  sensation.  Besides  this  effect,  heat  acts  variously 
upon  bodies  ;  it  melts  ice,  boils  water,  makes  metals  red-hot,  and 
so  forth. 

Two  theories  as  to  the  cause  of  heat  are  current  at  the  present 
time  ;  these  are  the  theory  of  emission,  and  the  theory  of  undula- 
tion, 

On  the  first  theory,  heat  is  caused  by  a  subtle  imponderable  fluid, 
which  surrounds  the  molecules  of  bodies,  and  which  can  pass  from 
one  body  to  another.  These  heat  atmospheres,  which  thus  surround 
the  molecules,  exert  a  repelling  influence  on  each  other,  in  conse- 
quence of  which  heat  acts  in  opposition  to  the  force  of  cohesion. 
The  entrance  of  this  substance  into  our  bodies  produces  the  sensa- 
tion of  warmth,  its  egress  the  sensation  of  cold. 

On  the  second  hypothesis  the  heat  of  a  body  is  caused  by  an 
oscillating  or  vibratory  motion  of  its  material  particles,  and  the 
hottest  bodies  are  those  in  which  the  vibrations  have  the  greatest 
velocity  and  the  greatest  amplitude.  Hence,  on  this  view,  heat  is 
not  a  substance,  but  a  condition  of  matter,  and  a  condition  which 
can  be  transferred  from  one  body  to  another.  It  is  also  assumed 
that  there  is  an  imponderable  elastic  ether,  which  pervades  all 
bodies  and  infinite  space,  and  is  capable  of  transmitting  a  vibratory 
motion  with  great  velocity.  A  rapid  vibratory  motion  of  this  ether 
produces  heat,  just  as  sound  is  produced  by  a  vibratory  motion  ot 


1 90  On  Heat.  [194- 

atmospheric  air,  and  the  transference  of  heat  from  one  body  to 
another  is  effected  by  the  intervention  of  this  ether. 

This  hypothesis  is  now  admitted  by  the  most  distinguished 
physicists  ;  it  affords  a  better  explanation  of  the  phenomena  of 
heat  than  any  other  theory,  and  it  reveals  an  intimate  connection 
between  heat  and  light.  In  accordance  with  it,  heat  is  a  form 
of  motion  ;  and  it  will  hereafter  be  shown  that  heat  may  be  con- 
verted into  motion,  and,  conversely,  motion  may  be  converted  into 
heat. 

Although  the  undulatory  theory  of  heat  is  the  correct  one,  the 
one,  that  is,  which  best^  explains  and  accounts  for  the  greatest 
number  of  facts,  yet  it  may  be  sometimes  convenient  to  use 
language  which  is  based  on  the  older  hypothesis.  Thus,  in  speaking, 
of  a  body  becoming  heated  or  cooled,  we  say  that  it  gains  or  loses 
heat :  in  reality,  the  motion  of  the  particles  is  increased  or 
diminished. 

In  what  follows,  however,  the  phenomena  of  heat  will  be  con- 
sidered, as  far  as  possible,  independently  of  either  hypothesis. 

195.  General  effects  of  heat. — The  general  action  of  heat  upon 
bodies  is  to  develope  a  repulsive  force  between  their  molecules 
which  is  continually  struggling  with  molecular  attraction.  Under 
its  influence,  therefore,  bodies  tend  to  expand — that  is,  to  assume  a 
greater  volume. 

All  bodies  expand  by  the  action  of  heat.  As  a  general  rule 
gases  are  the  most  expansible,  then  liquids,  and,  lastly,  solids.  The 
expansion  of  bodies  by  heat  is  thus  a  new  general  property  to  be 
added  to  those  already  studied. 

The  action  of  heat  upon  bodies  is  not  merely  to  expand  them  ; 
when  accumulated  in  sufficient  quantity,  bodies  first  lose  their 
solidity  and  become  somewhat  softer  ;  then,  as  the  heat  still  in- 
creases, the  force  of  repulsion  balances  molecular  attraction,  and 
then  bodies  liquefy.  Wax,  resin,  sulphur  thus  pass  readily  from 
the  solid  to  the  liquid  state  ;  heat  thus  produces  in  solids  a  change 
of  state  of  aggregation.  But  in  liquids  it  also  produces  a  similar 
change.  When  bodies  are  heated  they  first  expand  ;  heated  still 
more  their  molecular  attraction  is  again  overcome  by  the  force  of 
repulsion,  and  bodies  are  then  changed  into  aeriform  liquids  called 
vapours. 

If,  instead  of  becoming  accumulated  in  bodies,  heat  is  given  out, 
that  is,  if  bodies  are  cooled  instead  of  being  heated,  the  opposite 
phenomena  are  produced  :  the  molecules  come  nearer  each  other, 


-196]  Expansion.  191 

the  volume  of  the  pores  diminishes,  and  hence  that  of  the  body, 
which  is  expressed  by  saying  that  the  body  contracts.  By  cooling, 
vapours,  losing  their  elastic  force,  revert  to  the  liquid  state  ;  and 
liquids  themselves,  by  the  same  process,  gradually  return  to  the 
solid  state.  Thus  water  changes  into  ice,  and  mercury  becomes  as 
hard  as  lead. 

Thus,  according  as  heat  accumulates  in,  or  is  dissipated  by, 
bodies,  two  physical  effects  may  be  produced  :  i,  Changes  in  volume, 
consisting  in  expansions  and  contractions.  2.  Changes  of  condi- 
tion, that  is,  the  change  of  solids  into  liquids,  of  liquids  into  vapours, 
and  conversely.  We  shall  first  discuss  the  expansion  of  bodies,  and 
afterwards  their  changes  of  state. 

196.  Expansion. — All  bodies  are  expanded  by  heat,  but  to  very 
different  extents.  Gases  are  most  expansible,  then  liquids,  and 
after  them  solids. 

In  solids  which  have  definite  figures,  we  can  either  consider  the 
expansion  in  one  dimension,  or  the  linear  expansion  ;  in  two  dimen- 
sions, the  superficial  expansion,  or  in  three  dimensions,  the  cubical 
expansion  or  the  expansion  of  volume,  although  one  of  these  never 
takes  place  without  the  other.  As  liquids  and  gases  have  no 
definite  shapes,  we  can  only  consider  the  alterations  of  volume,  which 
they  undergo. 


Fig.  169. 

To  show  the  linear  expansion  of  solids,  the  apparatus  represented 
in  fig.  169  may  be  used..    A  metal  rod,  A,  is  fixed  at  one  end  by 


192 


On  Heat. 


[196- 


a  screw,  B,  while  the  other  end  presses  against  the  short  arm,  C, 
of  an  index,  D,  which  moves  on  a  scale.  Below  the  rod,  A,  there  is 
a  sort  of  cylindrical  lamp  in  which  spirit  is  burned.  The  needle, 
D,  is  at  first  at  the  zero  point,  but,  as  the  rod  becomes  heated, 
the  needle  moves  along  the  scale,  which  shows  that  the  short  arm, 
C,  of  the  lever  is  slightly  displaced,  pushed  by  the  rod,  A,  as  it 
expands. 

It  will  be  observed  that  if  rods  of  different  metals  are  used,  the 
index  will  be  moved  to  different  extents,  showing  that  their  expan- 
sibility differs.  Thus  it  will  be  found  that  brass  is  more  expansible 
than  iron,  or  steel. 

The  cubical  expansion  of  solids  is 'shown   by  a  Gravcsande's 

ring.  It  consists  of  a  brass  ball 
a  (fig.  170),  which  at  .the  ordin- 
ary temperature  passes  freely 
through  a  ring,  ;/z,  almost  of  the 
same  diameter.  But  when  the 
ball  has  been  heated,  it  expands 
and  no  longer  passes  through 
the  ring.  It  does  so,  however, 
on  reverting  to  its  original  tem- 
perature. The  expansibility  of 
liquids  and  gases,  which  is  far 
greater  than  that  of  solids,  is 
easily  shown.  For  a  liquid  a 
glass  tube  with  a  bulb  at  one 
end  may  be  u.sed  (fig.  171), 
Fig.  170.  which  is  filled  with  some  liquid, 

coloured  alcohol  or  mercury,  for 

instance.  When  the  bulb  is  gently  heated,  by  placing  it  in  tepid 
water,  for  example,  the  column  of  liquid  is  seen  to  rise  consider- 
ably in  the  tube  ;  thus  from  a  to  b. 

The  experiment  may  be  made  in  a  similar  manner  with  gases  ; 
yet  as  they  are  far  more  expansible  than  liquids,  a  long  tube  bent 
twice  maybe  fused  to  the  bulb  tube,  as  represented  in  fig.  172. 
An  index  of  mercury,  m,  is  introduced  in  the  tube,  which  is  effected 
by  gently  heating  the  bulb  so  as  to  expel  some  of  the  air  ;  a  drop 
of  mercury  being  then  placed  in  the  funnel,  a,  on  cooling  the  air 
in  the  bulb  and  the  tube  contracts,  and  the  pressure  of  the  atmo- 
sphere forces  the  droplet  to  ;;/  for  instance.  The  apparatus  being 
thus  arranged,  if  the  bulb  is  held  in  the  hand  for  a  few  moments, 


-198] 


Thermometers. 


193 


the  enclosed  air  expands  sufficiently  to  force  the  index  from  m  to 
«,  an  expansion  which  is  far  greater  than  in  the  case  of  liquids. 

It  will  thus  be  seen  that  the  general 
effect  of  heat  upon  bodies  is  to  expand 
them.  Yet  this  only  applies  to  bodies 
which,  like  the  metals,  glass,  etc.  do  not 
absorb  moisture.  Bodies  which  absorb 
moisture,  such  as  wood,  paper,  clay, 
undergo  a  contraction  when  heated, 
owing  to  the  increase  of  temperature  ex- 
pelling moisture  from  their  pores.  Thus 
a  moist  sheet  of  paper  placed  before  the 
fire  coils  up  on  the  heated  side.  Coopers, 
too,  to  curve  the  staves  of  barrels,  heat 
them  on  one  side,  by  lighting  a  fire  in 
the  inside  of  the  barrel  when  the  staves 
are  placed  close  together.  The  part 
turned  towards  the  fire  contracts  in  dry- 
ing, and  curves  on  the  side  exposed  to 
the  action  of  heat. 


MEASUREMENT  OF  TEMPERATURES. 
THERMOMETRY. 

197.  Temperature.  —  The    tempe- 
rature or  hotness  of  a  body  may  be  de- 
fined as  being  the  greater  or  less  extent 
to  which  it  tends  to  impart  sensible  heat 

to  other  bodies.  The  temperature  of  any  particular  body  is  varied, 
by  adding  to  it  or  withdrawing  from  it  a  certain  amount  of  sensible 
heat.  The  temperature  of  a  body  must  not  be  confounded  with 
the  quantity  of  heat  it  possesses  ;  a  body  may  have  a  high  tempe- 
rature and  yet  have  a  very  small  quantity  of  heat,  and  conversely 
a  low  temperature  may  yet  possess  a  large  amount  of  heat.  If  a 
cup  of  water  be  taken  from  a  bucketful,  both  will  indicate  the  same 
temperature,  yet  the  quantities  of  heat  they  possess  will  be  different. 
The  subject  of  the  quantity  of  heat  will  be  afterwards  more  fully 
explained  in  the  chapter  on  SPECIFIC  HEAT. 

198.  Thermometers. — Thermometers  are  instruments  for  mea- 
suring temperatures.     Owing  to  the  imperfections  of  our  senses  we 
are  unable  to  measure  temperatures  by  the  sensations  of  heat  or 

O 


194 


On  Heat. 


[198- 


cold  which  they  produce  in  us,  and  for  this  purpose  recourse  must 
l>e  had  to  the  physical  effects  of  heat  upon  bodies.  The  most 
accurate  and  the  most  convenient  are  the  expansive  effects.  Solids, 
having  but  little  expansibility,  can  only  be  used  to  examine  large 
intervals  of  temperature  ;  gases,  on  the  other  hand,  are  very  ex- 
pansible, and  only  serve  to  measure  small  alterations  of  temperature. 
They  are,  moreover,  affected  by  changes  of  atmospheric  pressure. 
For  these  reasons,  liquids  are  best  suited  for  the  construction  of 

thermometers.  Mercury  and  alcohol 
are  the  only  ones  used — the  former 
because  its  expansion  is  regular,  and 
it  only  boils  at  a  very  high  temperature, 
and  the  latter  because  it  does  not 
solidify  at  the  greatest  known  cold. 

The  mercurial  thermometer  is  the 
most  extensively  used.  It  consists  of 
a  capillary  glass  tube,  at  the  end  of 
which  is  blown  the  bulb,  a  cylindrical 
or  spherical  reservoir  (fig.  173).  Both 
the  bulb  and  a  part  of  the  stem  are 
filled  with  mercury,  and  the  expansion 
is  measured  by  a  scale  graduated  either 
on  the  stem  itself  (fig.  178),  or  on  a 
frame  to  which  it  is  attached  (fig. 

177). 

The  filling  of  the  tube  with  mercury 
is  effected  by  fusing  to  the  tube  a  small 
funnel  as  shown  in  fig.  173.  In  this  is 
placed  a  small  quantity  of  mercury, 
and  the  bulb  is  then  gently  heated  by 
a  spirit  lamp.  The  expanded  air  par- 
tially escapes  by  the  funnel,  and  on 
cooling,  the  air  which  remains  con- 
tracts, and  a  portion  of  the  mercury 
passes  into  the  bulb.  The  bulb  is  then 
again  warmed,  and  allowed  to  cool,  a 


Fig.  173- 

fresh  quan 


Fig.  174. 


tity  of  mercury  enters,  and  so  on,  until  the  bulb  and 
part  of  the  tube  are  full  of  mercury.  The  mercury  is  then  heated  to 
boiling  ;  the  mercurial  vapours  in  escaping  carry  with  them  the  air 
and  moisture  which  remain  in  the  tube.  The  tube  being  full  of  the 
expanded  mercury  and  of  mercurial  vapour,  is  hermetically  sealed 


-199] 


Graduation  of  tJie  Thermo? faeter. 


195 


at  one  end.     When  the  thermometer  is  cold  the  mercury  ought  to 
fill  the  bulb  and  a  portion  of  the  stem. 

199.  Graduation  of  the  thermometer. — The  thermometer 
having  been  filled,  as  has  just  been  described,  whenever  the  tem- 
perature rises  or  sinks,  the  mercury  rises  or  sinks  in  the  stem,  and 
these  variations  furnish  a  means  of  measuring  temperatures.  For 
this  purpose  a  graduated  scale 
must  be  constructed  along  the 
stem.  In  graduating  the  scale 
two  points  must  be  fixed,  which 
represent  identical  temperatures 
and  which  can  always  be  easily 
produced. 

Experiment  has  shown  that 
ice  always  melts  at  the  same 
point  whatever  be  the  degree 
of  heat,  and  that  distilled  water 
under  the  same  pressure,  and  in 
a  vessel  of  the  same  kind,  always 
boils  at  the  same  temperature. 
Consequently,  for  the  first  fixed 
point,  or  zero,  the  temperature 
of  melting  ice  has  been  taken  ; 
and,  for  a  second  fixed  point, 
the  temperature  of  boiling  water 
in  a  metallic  vessel  under  the 
normal  atmospheric  pressure  of 
30  inches. 

This  interval  of  temperature, 
that  is,  the  range  from  zero  to 
the  boiling  point,  is  taken  as  the 
unit  for  comparing  tempera- 
tures ;  just  as  a  certain  length,  a 
foot  or  a  yard  for  instance,  is  used 
as  a  basis  for  comparing  lengths.  Flg>  I75- 

To  obtain  zero,  snow  or  pounded  ice  is  placed  in  a  vessel,  in 
the  bottom  of  which  is  an  aperture  by  which  water  escapes  (fig. 
175).  The  bulb  and  a  part  of  the  stem  of  the  thermometer  are 
immersed  in  ihis  for  about  a  quarter  of  an  hour  ;  the  mercury 
sinks,  and  the  level  at  which  it  finally  rests,  t  for  instance,  is  marked 
by  tying  a  piece  of  thread  round  the  stem. 

O  2 


196 


On  Heat. 


[199- 


The  second  fixed  point  is  determined  by  means  of  the  apparatus 
represented  in  fig.  176.     It  consists  of  a  tin-plate  vessel  containing 


Fig.  176. 


Fig.  177- 


distilled  water,  in  the  lid  of  which  is  a  long  tube.     The  thermo- 
meter is  placed  in  this  by  means  of  a  cork,  and  the  water  heated 


-200]      Construction  of  the  Thermometer  Scale.  197 

to  boiling.  The  thermometer  is  thus  surrounded  by  steam,  which, 
liberated  from  the  liquid  escapes  by  the  lateral  apertures.  This 
steam  is  at  the  same  temperature  as  the  water  from  which  it  is 
liberated,  and,  when  the  mercury  is  stationary,  a  second  mark  is 
made  upon  the  stem. 

200.  Construction  of  the  scale.— Just  as  the  foot-rule  which  is 
adopted  as  the  unit  of  comparison  for  length  is  divided  into  a 
number  of  equal  divisions  called  inches,  for  the  purpose 
of  having  a  smaller  unit  of  comparison,  so  likewise  the 
unit  of  comparison  of  temperatures,  the  range  from  zero 
to  the  boiling  point,  must  be  divided  into  a  number  of 
parts  of  equal  capacity  called  degrees.  There  are  three 
modes  in  which  this  is  done.  On  the  Continent,  and  more 
especially  in  France,  this  space  is  divided  into  100  parts, 
and  this  division  is  called  the  Centigrade  or  Celsius  scale  ; 
the  latter  being  the  name  of  the  inventor.  The  Centigrade 
thermometer  is  almost  exclusively  adopted  in  foreign 
scientific  works,  and  as  its  use  is  gradually  extending  in 
this  country,  it  has  been  and  will  be  adopted  in  this  book. 

The  degrees  are  designated  by  a  small  cipher  placed 
a  little  above  on  the  right  of  the  number  which  marks 
the  temperature,  and  to  indicate  temperatures  below  zero 
the  minus  sign  is  placed  before  them.  Thus  -  15° 
signifies  15  degrees  below  zero. 

In  accurate  thermometers  the  scale  is  marked  on  the 
stem  itself  (fig.  178).  It  cannot  be  displaced,  and  its 
length  remains  fixed,  as  glass  has  very  little  expansi- 
bility. This  is  effected  by  covering  the  stem  with  a  thin 
layer  of  wax,  and  then  marking  the  divisions  of  the  scale, 
as  well  as  the  corresponding  numbers,  with  a  steel  point. 
The  thermometer  is  then  exposed  for  about  ten  minutes 
to  the  vapours  of  a  substance  called  hydrofluoric  acid, 
which  attacks  the  glass  where  the  wax  has  been  removed. 
The  rest  of  the  wax  is  then  removed,  and  the  stem  is 
found  to  be  permanently  etched. 

Scales  are  also  constructed  on  plates  of  ivory,  wood,    Fig7i78. 
or  metal,  against  which  the  stem  is  placed.     Fig.  177, 
represents  a  mercury  thermometer   mounted  on  ivory;  its  scale 
extends  from  20  degrees  below  zero  to  1 10  degrees  above. 

Besides  the  Centigrade  scale  two  others  are  frequently  used — 
Fahrenheit's  scale  and  Reaumur's  scale. 


198  0//  #htf.  [200- 

In  Reaumur's  scale  the  fixed  points  are  the  same  as  on  the 
Centigrade  scale,  but  the  distance  between  them  is  divided  into  So 
degrees  instead  of  into  icn.  That  is  to  say,  80  degrees  Reaumur 
are  equal  to  ico  degrees  Centigrade  ;  one  degree  Reaumur  is 
equal  to  ^  or  f  of  a  degree  Centigrade,  and  one  degree  Centigrade 
equals  T8~  or  *  degree  Reaumur.  Consequently  to  convert  any 
number  of  Reaumur  degrees  into  Centigrade  degrees  (20  for  ex- 
ample), it  is  merely  necessary  to  multiply  them  by  f  (which  gives 
25).  Similarly,  Centigrade  degrees  are  converted  into  Reaumur's 
by  multiplying  them  by  f. 

The  thermometric  scale  invented  by  Fahrenheit  in  1714  is  still 
much  used  in  England,  and  also  in  Holland  and  North  America. 
The  higher  fixed  point  is  like  that  of  the  other  scales,  the  tempera*- 
ture  of  boiling  water,  but  the  null-point  or  zero  is  the  temperature 
obtained  by  mixing  equal  weights  of  sal-ammoniac  and  snow,  and 
the  interval  between  the  two  points  is  divided  into  212  degrees. 
The  zero  was  selected  because  the  temperature  was  the  lowest  then 
known,  and  was  erroneously  thought  to  represent  absolute  cold. 
When  Fahrenheit's  thermometer  is  placed  in  melting  ice  it  stands  at 
32  degrees,  and,  therefore,  100  degrees  on  the  Centigrade  scale  are 
equal  to  180  degrees  on  the  Fahrenheit  scale,  and  thus  i  degree 
Centigrade  is  equal  to  f  degree  Fahrenheit,  and  conversely  I  degree 
Fahrenheit  is  equal  to  f  of  a  degree  Centigrade. 

If  it  be  required  to  convert  a  certain  number  of  Fahrenheit  de- 
grees (95  for  example)  into  Centigrade  degrees,  the  number  32 
must  be  first  substracted,  in  order  that  the  degrees  may  count  from 
the  same  point  of  the  scale.  The  remainder  in  the  example  is  thus 
63,  and  as  i  degree  Fahrenheit  is  equal  to  f  of  a  degree  Centigrade, 
63  degrees  are  equal  10.63  x  f  or  35  degrees  Centigrade. 

If  F  be  the  given  temperature  in  Fahrenheit's  degrees  and  C 
the  corresponding  temperature  in  Centigrade  degrees,  the  former 
may  be  converted  into  the  latter  by  means  of  the  formula 


(F-32) 


and  conversely,  Centigrade  degrees  may  be  converted  into  Fahren- 
heit by  means  of  the  formula 


These  formulae  are  applicable  to  all  temperatures  of  the  two  scales, 
provided  the  signs  are  taken  into  account.     Thus,  to  convert  the 


-202] 


A  Icohol  Thermometer. 


199 


temperature  of  5  degrees  Fahrenheit  into  Centigrade  degrees  we 
have 


In  like  manner  we  have  for  converting  Reaumur's  into  Fahren- 
heit's degrees  the  formula 


and  conversely,  for  changing  Fahrenheit's  into  Reaumur's  degrees 
the  formula 

(F-32)|  =  R. 

201.  Alcohol  thermometer.  —  The  alcohol  thermometer  differs 
from   the  mercurial   thermometer   in    being   filled   with   coloured 
alcohol.     But  as  the  expansion  of  liquids  is  less  regular  in  propor- 
tion as  they  are  near  the  boiling  point,  alcohol,  which  boils  at  78°  C, 
expands  very  irregularly.     Hence,  alcohol  thermometers  are  usually 
graduated  by  placing  them  in  baths  at  different  temperatures  to- 
gether  with    a    standard 

mercurial  thermometer. 

The  filling  is  effected 
by  gently  heating  the  bulb, 
so  as  to  expel  a  certain 
quantity  of  air,  then  in- 
verting it  and  plunging  the 
open  end  into  alcohol  (fig. 
179).  The  interior  air 
contracts  on  cooling,  and 
the  atmospheric  pressure 
raises  the  alcohol  in  the 
tube  and  in  the  bulb.  It 
does  not  at  first  fill  it 
completely,  for  some  air 
remains  ;  but  the  alcohol  is  then  boiled,  and  its  vapours  expel 
all  the  air  ;  the  tube  is  then  again  inverted  and  placed  in  alcohol, 
and  now  the  instrument  fills  completely.  The  further  construction 
resembles  that  of  a  mercurial  thermometer. 

202.  Limits  to  the  employment  of  mercurial  thermometers. 
—  Of  all  thermometers  in  which  liquids  are  used,   the  one  with 
mercury  is  the  most  useful,  because  this  liquid  expands  most  re- 
gularly, and  is  easily  obtained  pure,  and  because  its  expansion  be- 
tween —  36°  and  100°  is  regular,  that  is,  proportional  to  the  degree 


Fig.  179. 


2OO 


On  Heat.  [202- 

of  heat.  It  also  has  the  advantage  of  having  a  very  low  specific 
heat.  But  for  temperatures  below  -  36°  C.  the  alcohol  thermometer 
must  be  used,  for  mercury  solidifies  at  — 40°  C.  to  a  mass  like  lead. 
Above  100  degrees  the  coefficient  of  expansion  increases  'and  the 
indications  of  the  mercurial  thermometers  are  only  approximate, 
the  error  amounting  sometimes  to  several  degrees.  Mercurial 
thermometers  also  cannot  be  used  for  temperatures  above  350°,  for 
this  is  the  boiling  point  of  mercury.  \ 

Observations  by  means  of  the  thermometer.  In  taking  the  tem- 
perature of  a  room,  the  thermometer  is  usually  suspended  against 
the  wall.  This  may,  however,  give  rise  to  an  error  of  several  de- 
grees ;  for  if  the  wall  communicates  with  the  outside,  and  especially  if 
it  has  a  northern  aspect,  it  will,  generally  speaking,  be  colder  than 
the  air  in  the  room,  and  will  communicate  to  the  thermometer  too 
low  a  temperature.  On  the  other  hand  it  may  happen  that  the  wall 
becomes  too  much  heated  by  the  sun's  rays,  or  by  chimney  flues, 
and  then  the  thermometer  will  be  too  high.  The  only  way  to 
obtain  with  accuracy  the  temperature  of  the  air  in  a  room  is  to 
suspend  the  thermometer  by  a  string  in 
the  centre  at  a  distance  from  any  object 
which  might  raise  or  lower  its  tempera- 
ture. The  same  remark  applies  to  the 
determination  of  the  temperature  of  the 
atmosphere  ;  the  thermometer  must  be 
suspended  in  the  open  air,  in  the  shade, 
and  not  against  a  wall. 

203.  Leslie's  differential  thermo- 
meter.— Sir  John  Leslie  constructed  a 
thermometer  for  showing  the  difference 
in  temperature  of  two  neighbouring  places, 
from  which  it  has  received  the  name 
differential  thermometer.  It  consists  of 
two  glass  bulbs  containing  air,  and  joined 
by  a  bent  glass  tube  of  small  diameter  fixed 
on  a  frame  (fig.  180).  Before  the  apparatus 
is  sealed,  a  coloured  liquid  is  introduced 
in  sufficient  quantity  to  fill  the  horizontal 
part  of  the  tube  and  about  half  the  vertical  legs.  It  is  im- 
portant to  use  a  liquid  which  does  not  give  off  vapours  at  ordinary 
temperatures,  and  dilute  sulphuric  acid  coloured  with  litmus  is 
generally  preferred.  The  apparatus  being  closed,  the  air  is  passed 


Fig.  i 


—204]       Maximum  and  Minimum  Tliermometers.       201 

from  one  bulb  into  the  other,  by  heating  them  unequally,  until  the 
level  of  the  liquid  is  the  same  in  both  branches.  A  zero  is  marked 
at  each  end  of  the  liquid  column.  To  graduate  the  apparatus, 
one  of  the  bulbs  is  raised  to  a  temperature  10°  higher  than  the 
other.  The  air  of  the  first  is  expanded  and  causes  the  column  of 
liquid,  ba,  to  rise  in  the  other  leg.  When  the  column  is  stationary 
the  number  10  is  marked  on  each  side  at  the  level  of  the  liquid,  the 
distance  between  zero  and  10  being  divided  into  10  equal  parts, 
both  above  and  below  zero,  on  each  leg. 

204.  Rutherford's  maximum  and  minimum  thermometers. — 
It  is  necessary,  in  meteorological  observations,  to  know  the  highest 
temperature  of  the  day,  and  the  lowest  temperature  of  the  night. 
Ordinary  thermometers  could  only  give  these  indications  by  a  con- 
tinuous observation,  which  would  be  impracticable.  Several  in- 
struments have  accordingly  been  invented  for  this  purpose,  the 
simplest  of  which  is  Rutherford's.  On  a  rectangular  piece  of  plate 
glass  (fig.  181)  two  thermometers  are  fixed,  whose  stems  are  bent 

i°K 

Iwte 


40  00  20 


Fig.  181. 

horizontally.  The  one,  A,  is  a  mercurial,  and  the  other,  B,  an 
alcohol  thermometer.  In  A  there  is  a  small  piece  of  iron  wire,  A, 
moving  freely  in'  the  tube,  which  serves  as  an  index.  The  ther- 
mometer being  placed  horizontally,  when  the  temperature  rises  the 
mercury  pushes  the  index  before  it.  But  as  soon  as  the  mercury 
contracts,  the  index  remains  in  that  part  of  the  tube  to  which  it  has 
been  moved,  for  there  is  no  adhesion  between  the  iron  and  the 
mercury.  In  this  way  the  index  registers  the  highest  temperature 
which  has  been  obtained  ;  in  the  figure  this  is  31°.  In  the  minimum 
thermometer  there  is  a  small  hollow  glass  tube  which  serves  as 
index.  When  it  is  at  the  end  of  the  column  of  liquid,  and  the  tem- 
perature falls,  the  column  contracts  and  carries  the  index  with  it, 


202  On  Heat.  [204- 

in  consequence  of  adhesion,  until  it  has  reached  the  greatest  con- 
traction. When  the  temperature  rises,  the  alcohol  expands,  and 
passing  between  the  sides  of  the  tube  and  the  index  does  not  dis- 
place B.  The  position  of  the  index  gives  therefore  the  lowest  tem- 
perature which  has  been  reached  :  in  the  figure  this  was  9^  degrees 
frelow  zero. 

205.  Pyrometers. — The  name  pyrometers  is  given  to  instru- 
ments for  measuring  temperatures  so  high  that  mercurial  ther- 
mometers could  not  be  used.  The  older  contrivances  for  this 
purpose,  Wedgwood's,  Daniell's  (which  in  principle  resembled  the 
apparatus  in  fig.  170),  Brongniart's,  etc.,  are  gone  entirely  out  of 
use.  None  of  them  gives  an  exact  measure  of  temperature. 


CHAPTER    II. 

RADIATION   OF   HEAT. 

206.  Radiant  heat. — If  we   stand  in  front  of  a  fire,  or  expose 
ourselves  to  the  sun's  heat,  we  experience  a  sensation  of  warmth 
which  is  not  due  to  the  temperature  of  the  air,  for  if  a  screen  be 
interposed,  the  sensation  immediately  disappears,  which  would  not 
be  the  case  if  the  surrounding  air  had  a  high  temperature.      Hence 
bodies  can  send  out  rays  which  excite  heat,  and  which  penetrate 
through  the  air  without  heating  it,  as  rays  of  light  through  trans- 
parent bodies.     Heat  thus  propagated  is  said  to  be  radiated ;  and 
we  shall  use  the  term  ray  of  heat,  or  thermal,  or  calorific  ray,  in  a 
similar  sense  to  that  in  which  we  use  the  term  ray  of  light,  or 
luminous  ray. 

We  shall  find  that  the  property  of  radiating  heat  is  not  confined 
to  incandescent  substances,  such  as  a  fire,  or  a  lamp,  or  a  red-hot 
ball,  but  that  bodies  of  all  temperatures  radiate  heat.  Thus  a 
bottle  full  of  hot  water  and  a  bottle  full  of  cold  water  both  emit  heat  ; 
the  first  emits  more  as  compared  with  the  second,  the  greater  the 
difference  of  temperature  between  the  two. 

207.  Laws  of  radiation. — The  radiation  of  heat  is  governed  by 
three  laws. 

I.  Radiation  takes  place  in  all  directions  round  a  body.  If  a 
thermometer  be  placed  in  different  positions  round  a  heated  body, 
it  indicates  everywhere  a  rise  in  temperature  ;  at  equal  distances 
from  the  source  of  heat  it  indicates  the  same  rise  in  temperature. 


-208]  Radiant  Heat.  203 

II.  Heat  is  propagated  in  a  right  line.    For,  if  a  screen  be  placed 
in  the  right  line  which  joins  the  source  of  heat  and  the  thermometer, 
so  as  to  stop  the  rays,  the  latter  is  not  affected. 

But  in  passing  obliquely  from  one  medium  into 
another,  as  from  air  into  a  glass,  calorific  like  luminous 
rays  become  deviated,  an  effect  known  as  refraction. 
The  laws  of  this  phenomenon  are  the  same  for  heat 
as  for  light,  and  they  will  be  more  fully  discussed 
under  the  latter  subject. 

III.  Radiant  heat  is  propagated  in  vacua  as  -well 
as  in  air.     This  is  demonstrated  by  the  following  ex- 
periment. 

In  the  bottom  .of  a  glass  flask  a  thermometer  is 
fixed  in  such  a  manner  that  its   bulb  occupies   the 
centre  of  the  flask  (fig.  182).     The  neck  of  the  flask 
is  then  carefully  narrowed  by  means  of  the  blow-pipe,      F;g.  l82. 
and  then  the  apparatus  having  been  suitably  attached 
to  an  air-pump  a  vacuum  is  produced  in  the  interior.     This  having 
been  done,  the  tube  is  sealed  at  the  narrow  part.      On  immersing 
this  apparatus  in  hot  water,  or  on  bringing  near  it  some  hot  char- 
coal, the  thermometer  is  at  once  seen  to  rise.     This  could  only 
be  due  to  radiation  through  the  vacuum  in  the  interior,  for  glass  is 
so  bad  a  conductor,  that  the  heat  could  not  travel  with  this  rapidity 
through  the  sides  of  the  flask,  and  the  stem  of  the  thermometer. 

208.  Causes  which  modify  the  intensity  of  radiant  heat. — 
The  intensity  of  radiant  heat  transmitted  to  us  by  heated  bodies 
depends  on  the  temperature  of  the  source  of  heat,  and  on  its  distance. 
The  corresponding  laws  may  be  thus  stated  : 

I.  The  intensity  of  radiant  heat  is  proportional  to  the  temperature 
of  the  source. 

I 1.  The  intensity  of  radiant  heat  is  inversely  as  the  square  of  the 
distance. 

The  first  law  is  demonstrated  by  placing  a  metal  box  containing 
water  at  10°,  20°,  or  30°,  successively  at  equal  distances  from  the 
bulb  of  a  differential  thermometer.  The  temperatures  indicated  by 
the  latter  are  then  found  to  be  in  the  same  ratio  as  those  of  the  box  : 
for  instance,  if  the  temperature  of  that  corresponding  to  the  box  at 
10°  be  2°,  those  of  the  others  will  be  4°  and  6°  respectively. 

The  second  law  is  demonstrated  experimentally  by  placing  the 
differential  thermometer  at  a  certain  distance  from  the  source  of 
heat,  a  yard  for  distance,  and  then  removing  it  to  double  the 


204  On  Heat.  [208- 

distance.  In  the  latter  case,  the  amount  of  heat  received  is  not 
one-half  but  one-quarter.  If  the  distance  be  three  yards  the  quan- 
tity of  heat  is  one-ninth,  and  so  forth. 

209.  Interchange  of  heat  among  all  bodies. — Owing  to  the 
radiation  which  is  continually  taking  place  in  all  directions  round  a 
body,  there  is  a  continual  interchange  of  heat.  If  the  bodies  are  all 
at  the  same  temperature,  each  one  sends  to  the  surrounding  ones  a 
quantity  equal  to  that  which  it  receives,  and  their  temperatures 
remain  stationary.  But  if  their  temperatures  are  unequal,  as  the 
hot  bodies  emit  more  heat  than  they  receive,  they  therefore  sink  in 
temperature  ;  while,  as  the  bodies  of  lower  temperatures  receive 
more  heat  than  they  emit,  their  temperature  rises  ;  thus  the  tempera- 
tures are  all  ultimately  equal.  The  radiation  does  not  stop  ;  it  goes 
on,  but  without  loss  or  gain  from  each  body,  and  this  condition  is 
accordingly  known  as  the  mobile  equilibrium  of  temperature. 

From  what  has  been  said  it  will  be  understood  that  bodies 
placed  in  our  rooms  all  tend  to  assume  a  uniform  temperature  ; 
generally  speaking  this  is  not  the  case,  for  many  causes  concur  in 
cooling  one  set,  and  in  heating  the  others.  Thus  bodies  placed 
near  a  wall  cooled  by  the  outer  air  find  a  cause  for  cooling.  Those, 
on  the  contrary,  which  are  at  the  top  of  the  room,  tend  to  acquire  a 
higher  temperature  ;  for  as  heated  air  rises  as  being  less  dense, 
the  layers  nearest  the  ceiling  are  always  hotter  than  the  lower  ones. 

From  this  continual  interchange  of  heat,  there  is  necessarily  a 
limit  to  the  cooling  of  bodies,  for  they  always  tend  to  resume,  on 
the  one  hand,  what  they  lose  on  the  other.  To  have  an  indefinite 
cooling,  a  body  should  be  suspended  in  space,  not  receiving  heat 
from  any  body.  As  it  would  then  lose  heat  without  acquiring  any, 
there  is  no  telling  to  what  extent  its.  temperature  would  sink. 


CHAPTER   III. 

REFLECTION   OF   HEAT.      REFLECTING,   ABSORBING,  AND 
EMISSIVE   POWERS. 

210.  Iiaw  of  the  reflection  of  heat. — When  the  heat  rays 
emitted  by  a  source  of  heat  fall  upon  the  surface  of  a  body,  they 
are  divided  generally  into  two  parts  ;  one,  which  passes  into  the 
mass  of  a  body  and  raises  the  temperature  ;  the  other,  which  darts 


-211] 


Reflection  of  Heat. 


205 


off  from  the  surface  like  an  elastic  ball  striking  against  a  hard  body  ; 
this  is  expressed  by  saying  that  these  rays  are  reflected.  Thus  let 
A  be  the  source  of  heat,  a  cubical  box  rilled  with  hot  water  (fig. 
183),  and  near  it  a  screen  which  does  not  allow  heat  to  pass,  but 
near  the  bottom  of  which  is  an  aperture.  If  behind  this  screen  a 
polished  surface  be  placed  on  which  the  rays  emitted  by  the  cube 
impinge,  and  beyond  this  again  a  differential  thermometer,  the 
latter  indicates  an  increase  of  temperature  when  one  of  its  bulbs  is 
so  placed  that  it  receives  the  rays  reflected  by  the  polished  body. 
In  this  experiment,  rays  like  AB  which  fall  on  the  reflecting  surface 


;  ^^-.^^jt-^fs: 


Fig.  183. 

are  called  incident  rays,  from  a  Latin  word  which  signifies  to  fall ; 
and  the  angle  of  incidence  is  not  the  angle  which  they  make  with 
the  reflecting  surface,  but  the  angle,  ABC,  which  they  make  with  a 
straight  line,  BC,  perpendicular  to  this  surface.  In  like  manner  the 
angle,  CBD,  which  the  reflected  rays  make  with  the  same  straight 
line,  is  called  the  angle  of  reflection. 

The  reflection  of  heat  is  always  subject  to  the  law,  that  the  angle 
of  reflection  is  equal  to  the  angle  of  incidence.  We  shall  subse- 
quently see  that  the  reflection  of  light  is  governed  by  the  same  law. 

211.  Reflection  of  heat  from  concave  mirrors. — The  effects  of 
the  reflection  of  heat  may  be  very  powerful  when  it  takes  place  from 
the  surface  of  concave  mirrors,  which  are  spherical  surfaces  of  glass 
or  of  metal.  These  mirrors  may  be  regarded  as  being  made  up  of 
an  infinite  number  of  extremely  small  planes  inclined  towards  each 
other  in  such  a  manner  as  to  determine  the  curvature.  From  the 
symmetrical  grouping  of  these  small  facets,  it  follows  that  when  a 
group  of  rays  fall  upon  a  concave  mirror,  these  rays,  in  obedience 


2O6 


On  Heat. 


[211- 


to  the  laws  of  reflection  coincide  in  a  single  point,  to  which  the 
name  focus  is  applied,  to  express  the  great  quantity  of  heat  which 
is  concentrated  there.  (321). 

In  treating  of  light  we  shall  discuss  in  detail  the  properties  of  the 
focus  in  concave  mirrors  ;  for  the  present  it  will  be  suffick  nt  to  de- 
scribe experiments  which  demonstrate  the  great  intensity  which 
radiant  heat  may  acquire  when  concentrated  in  these  points.  Fig. 
184  represents  an  experiment  which  is  frequently  made  in  physical 
lectures.  Two  reflectors,  A  and  B  (fig.  185),  are  arranged  at  a  dis- 
tance of  4  to  5  yards,  and  so  that  their  axes  coincide.  In  the  focus 
of  one  of  them,  A,  is  placed  a  small  basket,  ;/,  containing  a  red-hot 


Fig.   184. 


iron  ball.  In  the  focus  of  the  other,  B,  is  placed  an  imflammable 
body,  such  as  gun-cotton  or  phosphorus.  The  rays  emitted  from 
the  focus,  71,  are  first  reflected  from  the  mirror,  A,  in  a  direction 
parallel  to  the  axis  ;  and  impinging  on  the  other  mirror,  B,  are 
reflected  so  that  they  coincide  in  the  focus  ;//.  That  this  is  so,  is 
proved  by  the  fact  that  the  inflammable  substance  placed  in  this 
point  takes  fire,  which  is  not  the  case  if  it  is  above  or  below  it. 

The  same  effect  may  be  produced  by  the  sun's  rays.  For  this 
purpose  a  concave  reflector  is  so  placed  that  the  sun's  rays  strike 
directly  against  it  (fig.  185)  ;  if  then  a  combustible  substance, 
such  as  paper,  vyood,  cork,  etc.,  be  held  by  means  of  a  pincette  in 
the  focus,  these  bodies  are  seen  to  take  fire.  The  effect  produced 


-212] 


Reflecting  Power  of  Substances. 


207 


depends  on  the  magnitude  of  the  mirrors.  With  a  mirror  having 
an  aperture  of  6  feet,  that  is,  the  distance  from  one  edge  to  the 
other,  copper  and  silver  are  melted  in  a  few  minutes  ;  and  silicious 
stones  and  flints  are  softened  and  even  melted. 

In  consequence  of  the  high  temperatures  produced  in  the  foci  of 
concave  mirrors  and  of  the  facility  with  which  combustible  bodies 
may  be  ignited  there,  they  have  been  called  burning  mirrors. 
It  is  stated  that  Archimedes  burnt  the  Roman  vessels  before  Syra- 
cuse by  means  of  such  mirrors.  Buffon  constructed  burning 


Fig.  185. 

mirrors  of  such  power  as  to  prove  that  the  feat  attributed  to  Archi- 
medes was  possible.  The  mirrors  were  made  of  a  number  of  silvered 
plane  mirrors  about  8  inches  long  by  5  broad.  They  could  be 
turned  independently  of  each  other  in  such  a  manner  that  the  rays 
reflected  from  each  coincided  in  the  same  point.  With  128  mirrors 
and  a  hot  summer's  sun  Buftbn  ignited  a  plank  of  tarred  wood  at  a 
distance  of  70  yards. 

212.  Reflecting:  power  of  various  substances. — It  has  been 
seen  that  heat  which  falls  upon  a  body  is  always  divided  into  two 


208 


On  Heat. 


[212- 


parts,  one  which  is  reflected  on  the  surface,  and  the  other  which 
passes  into  the  mass  of  the  body,  and  raises  its  temperature.  The 
quantities  of  heat  thus  absorbed,  or  reflected,  vary  in  different  sub- 
stances ;  one  set  reflects  much  and  absorbs  little,  which  is  ex- 
pressed by  saying  that  they  have  a  great  reflecting  power  ;  others, 
on  the  contrary,  reflect  very  little  heat,  but  absorb  a  great  deal,  and 
are  therefore  spoken  of  as  havirg  great  absorbing  power.  It  is 
clear  that  these  properties  are  the  inverse  of  each  other,  for  every 
body  which  absorbs  much  heat  can  reflect  but  little,  and  conversely. 
In  order  to  compare  the  reflecting  powers  of  various  substances, 
Leslie  took  as  a  source  of  heat  a  tin  plate  cube  full  of  boiling  water, 
which  he  placed  in  front  of  a  concave  mirror  (fig.  186).  The  rays 


Fig.  186. 

emitted  from  this  towards  the  reflector  tended  after  reflection  to 
become  concentrated  on  the  focus  F.  In  front  of  this  were  placed 
successively  small  square  plates  of  paper,  glass,  metal,  in  short,  ot 
all  the  substances  whose  reflecting  power  was  to  be  examined.  As 
shown  in  the  drawing,  these  rays  after  a  first  reflection  from  the 
mirror,  were  reflected  a  second  time  from  these  plates,  and  finally 
impringed  against  the  bulb  of  a  differential  thermometer.  Now,  as 
in  this  experiment  the  source  of  heat  was  the  same,  as  was  also  the 
distance  from  the  reflector  ;  yet  the  thermometer  indicated  very 
various  degrees  of  heat  according  to  the  material  of  which  the 
small  plates  were  formed.  The  temperature  was  highest  when  the 


-213] 


A  bsorbing  Power. 


209 


plate  was  made  of  polished  brass,  which  metal  is  therefore  the  best  re- 
flector. The  reflecting  power  of  silver  is  only  T95  that  of  brass  ;  that 
of  tin  i  ;  of  glass  £5.  Water  and  lampblack  were  found  to  be  des- 
titute of  reflecting  power,  for  when  the  plates  were  coated  with 
lampblack,  or  moistened  with  water,  the  thermometer  indicated  no 
increase  in  temperature,  showing  that  it  received  no  heat. 

213.  Absorbing-  power. — In  order  to  compare  the  absorbent 
powers  of  various  substances,  Leslie  arranged  the  experiment  as 
shown  in  fig.  187.  The  source  of  heat  and  the  reflector  being  the 


Fig.  187. 

same  as  in  the  preceding  experiment,  the  differential  thermometer 
was  placed  in  the  focus,  where  it  received  directly  all  the  heat  re- 
flected by  the  mirror.  The  surface  of  the  focal  bulb  was  altered 
for  each  experiment  by  coating  it  successively  with  various  ma- 
terials, paper,  tinfoil,  gold,  silver,  copper,  and  leadfoil ;  it  was  also 
coated  with  a  thin  layer  of  lampblack  ;  it  was  moistened,  and  so 
on.  It  was  thus  found  that  when  the  focal  bulb  was  coated  with 
lampblack,  or  with  water,  the  thermometer  indicated  the  highest 
temperatures  ;  whence  it  was  concluded  that  lampblack  and  water 
have  the  greatest  absorbing  power.  The  lowest  temperature  was 
exhibited  when  the  bulb  was  coated  with  thin  metal  foil,  more 
especially  with  silver ;  thus  indicating  that  these  substances  absorb 
the  least  proportion  of  the  heat  which  is  of  the  temperature  of  boiling 
water  (216).  The  result  was  arrived  at  which  could  indeed  be  fore- 

p 


210  On  Heat.  [213- 

seen,  that  those  bodies  which  best  reflect  heat  absorb  it  least  ;  and 
that,    conversely  the  best  absorbents  are  the  worst  reflectors. 

214.  Emissive  power. — The   emissive  or   radiating  power   is 
the  property  bodies   have  of  emitting  more  or  less  easily  the  heat 
they  contain  ;  it  is  the  inverse  of  the  absorbing  power. 

Leslie  compared  the  emissive  powers  of  various  bodies  by  means 
of  the  apparatus  represented  in  fig.  187.  The  focal  bulb  of  the 
thermometer  was  left  uncoated,  and  the  various  substances  were 
applied  successively  to  the  sides  of  the  tin  cube.  One  of  them,  for 
instance,  was  left  in  its  ordinary  condition  ;  the  second  was  coated 
with  lampblack  ;  to  the  third  a  sheet  of  white  paper  was  fixed,  and 
to  the  fourth  a  glass  plate. 

Turning  first  of  all  the  blackened  face  towards  the  reflector,  the 
thermometer  indicated  a  considerable  increase  of  temperature,  thus 
showing  that  the  cube  sent  much  heat  towards  the  reflector.  Turning 
then  successively  the  other  faces  towards  the  reflector,  it  was  found 
that  the  paper  side  emitted  less  heat  than  the  blackened  face,  but 
more  than  the  glass  side,  which  in  turn  emitted  more  than  the  tin  side. 

Working  in  this  manner,  Leslie  found  that  lampblack  has  the 
greatest  emissive  power,  then  paper,  then  ordinary  glass,  then 
the  metals.  *  The  order  of  their  emissive  powers  is  thus  the  same 
as  that  of  their  absorbing  powers.  It  is  thus  concluded  that  bodies 
which  best  absorb  heat,  also  radiate  best ;  and  Dulong  and  Petit 
have  proved  that  for  each  substance  the  emissive  power  is  in  all 
cases  proportional  to  the  absorbing  powen 

215.  Causes   which   modify  the   reflecting1,  absorbing-,  and 
radiating-  powers. — As  the  radiating  and  absorbing  powers  are 
equal,  any  cause  which  affects  the  one   affects  the  other  also.     And 
as  the  reflecting  power  varies  in  an  inverse  manner,  whatever  in- 
creases it  diminishes  the  radiating  and  absorbing  powers,  and-z/zV^ 
versa. 

It  has  been  already  stated  that  these  different  powers  vary  with 
different  bodies,  and  that  metals  have  the  greatest  reflecting  power, 
and  lampblack  the  feeblest.  In  the  same  body  these  powers  are 
modified  by  the  degree  of  polish,  the  density,  the  thickness  of  the 
radiating  substance,  the  obliquity  of  the  incident  or  emitted  rays, 
and,  lastly,  by  the  nature  of  the  source  of  heat. 

It  has  been  assumed  usually  that  the  reflecting  power  increases 
with  the  polish  of  the  surface,  and  that  the  other  powers  diminish 
therewith.  But  Melloni  showed,  that  by  scratching  a  polished 
metallic  surface  its  reflecting  power  was  sometimes  diminished  and 
sometimes  increased.  This  phenomenon  he  attributed  to  the 


-216]  Different  Kinds  of  Heat.  211 

greater  or  less  density  of  the  reflecting  surface.  If  the  plate  had 
been  originally  hammered,  its  homogeneity  would  be  destroyed  by 
this  process,  the  molecules  would  be  closer  together  on  the  surface 
than  in  the  interior,  and  the  reflecting  power  would  be  increased. 
But  if  the  surface  is  scratched  the  internal  and  less  dense  mass 
becomes  exposed,  and  the  reflecting  power  diminished.  On  the 
contrary,  in  a  plate  which  has  not  been  hammered  and  which  is 
homogeneous,  the  reflecting  power  is  increased  wrfen  the  plate  is 
scratched,  because  the  density  at  the  surface  is  increased  by  the 
scratches. 

The  absorbing  power  varies  with  the  inclination  of  the  incident 
rays.  It  is  greatest  at  right  angles  ;  and  it  diminishes  in  propor- 
tion as  the  incident  rays  'deviate  from  the  perpendicular  direction. 
This  is  one  of  the  reasons  why  the  sun  is  hotter  in  summer  than 
in  winter,  because,  in  the  former  case,  the  solar  rays  are  less  oblique. 

The  radiating  power  of  gaseous  bodies  in  a  state  of  combustion 
is  very  weak,  as  is  seen  by  bringing  the  bulb  of  a  thermometer  near 
a  hydrogen  flame,  the  temperature  of  which  is  very  high.  But  if  a 
platinum  spiral  be  placed  in  this  flame,  it  assumes  the  temperature 
of  the  flame,  and  radiates  a  considerable  quantity  of  heat,  as  is  in- 
dicated by  the  thermometer.  It  is  for  an  analogous  reason,  that 
the  flames  of  oil  and  of  gas  lamps  radiate  more  than  a  hydrogen 
flame,  in  consequence  of  the  excess  of  carbon  which  they  contain,  and 
which,  not  being  entirely  burned,  becomes  incandescent  in  the  flame. 

The  absorbing  power  of  a  body  is  also  influenced  by  the  nature 
of  the  source  of  heat.  Thus,  for  the  same  quantity  of  heat  emitted, 
a  surface  coated  with  white  lead  absorbs  twice  as  much,  if  the  heat 
comes  from  a  cube  filled  with  hot  water,  as  it  does  if  the  heat  is 
that  of  a  lamp.  Lampblack,  on  the  contrary,  absorbs  the  same 
amount  of  heat  whatever  be  the  source. 

216.  Different  kinds  of  beat.  Diathermaneity. — Just  as 
different  substances  possess  the  power  of  allowing  the  rays  of 
light  to  pass  through  them  to  different  extents,  and  are  said  to  be 
more  or  less  transparent  (305),  so  also  modern  investigation  has 
shown  that  all  bodies  do  not  allow  the  rays  of  heat  to  traverse 
them  with  equal  facility,  and  are  therefore  said  to  be  more  or  less 
diathermanous.  Thus  the  metals  are  just  as  adiathermanous  for 
heat  rays  as  they  are  opaque  for  the  rays  of  light.  On  the  other 
hand  rock  salt  stands  in  the  same  relation  to  heat  rays  that  a 
perfectly  colourless  and  transparent  body,  such  as  glass,  does  to 
luminous  rays.  It  is  perfectly  diathermanous. 

p  2 


212  On  Heat.  [216- 

One  and  the  same  substance  may  be  diathermanous  to  varying 
extents  for  heat  from  different  sources.  Thus  colourless  glass 
allows  the  sun's  rays  to  pass  through  it  with  facility,  but  less  so  the 
heat  emitted  by  a  flame,  or  by  an  incandescent  body,  and  far  less 
again  the  heat  of  a  cube  filled  with  boiling  water,  which  is  known 
as  a  Leslie's  cube  (212).  Water  allows  the  solar  heat  to  traverse  it 
partially,  but  stops  the  obscure  heat  of  a  Leslie's  cube.  Again, 
alum  is  colourless  and  transparent  for  light,  but  almost  entirely  dia- 
thermanous for  obscure  rays. 

A  body  which  is  opaque  for  light  may  be  diathermanous  for 
certain  kinds  of  heat.  Thus,  a  solution  of  iodine  in  bisulphide  of 
carbon  is  perfectly  opaque  for  the  rays  of  light,  but  is  traversed 
by  obscure  heat  rays  with  facility. 

In  investigating  the  diathermaneity  of  bodies,  Melloni  used  the 
thermo-multiplier,  for  a  description  of  which  we  must  refer  to  Book 
VIII.  Chapter  xiii.  He  first  of  all  placed  the  thermo-multiplier  at 
a  certain  distance  from  a  source  of  heat,  and  having  observed  the 
deflection,  he  determined  to  what  extent  this  was  enfeebled  by  the 
interposition  of  various  bodies,  such  as  plates  of  glass,  alum,  and 
rock  salt.  In  like  manner  he  used  various  sources  of  heat,  for 
instance  the  sun,  an  oil  or  spirit  lamp,  an  ignited  spiral  of  platinum 
wire,  a  heated  blackened  metal  plate,  or  a  cube  filled  with  hot 
water,  and  he  concluded  that  there  are  different  kinds  of  heat  rays, 
or  different  colours  of  heat,  with  regard  to  which  various  diather- 
manous substances  behave  just  as  coloured  transparent  substances 
do  in  regard  to  different  kinds  of  light.  Thus  when  white  light 
traverses  red  glass,  only  the  red  rays  are  transmitted,  all  other  kinds 
being  absorbed.  If  this  red  light  falls  on  another  red  glass  it 
traverses  it  without  enfeeblement  ;  but  is  completely  absorbed  by 
blue  glass.  Similar  results  are  met  with  in  regard  to  the  rays  of 
heat. 

217.  Applications. — The  property  which  bodies  possess  of  ab- 
sorbing, emitting,  and  reflecting  heat,  meets  with  numerous  appli- 
cations in  domestic  economy  and  in  the  arts.  Leslie  stated  that 
white  bodies  reflect  heat  very  well,  and  absorb  very  little,  and  that 
the  contrary  is  the  case  with  black  substances.  This  principle  is 
not  universally  true,  as  Leslie  supposed  ;  for  example,  white  lead 
has  as  great  an  absorbing  power  for  non-luminous  rays  as  lamp- 
black. But  it  holds  good  in  regard  to  absorbents  like  cloth,  cotton, 
wool,  and  other  organic  substances  when  exposed  to  luminous  heat, 
such  as  that  of  the  sun's  rays.  Accordingly,  the  most  suitable 


-218]  Conductivity  of  Solids.  213 

coloured  clothing  for  summer  is  just  that  which  experience  has 
taught  us  to  use,  namely,  white,  for  it  absorbs  less  of  the  sun's 
rays  than  black  clothing,  and  hence  feels  cooler. 

The  polished  fire-irons  before  a  fire  are  cold,  whilst  the  black 
fender  is  often  unbearably  hot.  If  a  liquid  is  to  be  kept  hot  as 
long  as  possible,  it  must  be  placed  in  a  brightly  polished  metallic 
vessel,  for  then,  the  emissive  power  being  less,  the  cooling  is  slower. 
It  is  for  this  reason  advantageous  that  the  steam  pipes,  etc.,  of 
locomotives  should  be  kept  brightly  polished. 

Snow  is  a  powerful  reflector,  and,  therefore,  neither  absorbs  nor 
emits  much  heat  ;  owing  to  its  small  emissive  power  it  protects 
from  cold  the  ground  and  the  plants  which  it  covers  ;  and  owing 
to  its  small  absorbing  power  it  melts  but  slowly  during  a  thaw. 
A  branch  of  a  tree,  a  bar  of  metal,  a  stone  in  the  midst  of  a  mass 
of  snow,  accelerate  the  fusion  by  the  heat  they  absorb,  and  which 
they  radiate  about  them. 

In  the  Alps  the  mountaineers  accelerate  the  fusion  of  the  snow 
by  covering  it  with  earth,  which  increases  the  absorbing  power. 

Metal  cooking  vessels  should  be  black  and  rough  on  the  outside, 
for  then  their  absorbing  power  is  greater  and  they  become  heated 
more  rapidly.  Their  bright  and  polished  surface  is  purchased  at 
the  expense  of  combustible.  This  is  what  is  seen  in  vessels  of 
silver  and  of  white  procelain.  In  common  unglazed  earthenware 
liquids  are  more  rapidly  heated,  but  also  more  rapidly  cooled. 

It  is  observed  that  the  ripening  of  grapes  and  other  fruits  is  ac- 
celerated when  they  are  placed  in  contact  with  a  black  wall  (mortar 
mixed  with  lampblack).  This  arises  from  the  fact,  that  from  the 
great  emissive  power  of  the  wall,  as  well  as  from  its  great  absorbing 
power,  it  becomes  more  highly  heated  under  the  influence  of  the 
sun,  and  gives  up  more  to  the  fruit. 

Glass  is  used  for  fire-screens,  for  while  it  allows  the  cheerful 
light  of  the  fire  to  pass,  it  stops  most  of  the  heat. 


CHAPTER   IV. 

CONDUCTING   POWER  OF   BODIES. 

2 1 8.  Conductivity  of  solids, — In  the  phenomena  of  radiation 
which  have  been  considered,  heat  is  transmitted  from  one  body  to 
another  through  space,  without  raising  the  temperature  of  the 


214 


On  Heat. 


[218- 


medium  through  which  it  passes.  It  may  also  be  propagated 
through  the  mass  of  a  body  by  an  internal  radiation  from  molecule 
to' molecule.  This  internal  propagation  in  the  mass  of  a  body  is 
called  conductivity  ;  and  good  conductors  are  those  bodies  which 
readily  transmit  heat  in  their  mass,  while  those  through  which  it 
passes  with  difficulty  are  called  bad  conductors. 

Organic  substances  conduct  heat  badly.  De  la  Rive  and  De 
Candolle  have  shown  that  woods  conduct  better  in  the  direction  of 
their  fibres  than  in  a  transverse  direction  ;  and  have  remarked  upon 
the  influence  which  this  feeble  conducting  power,  in  a  transverse 
direction,  exerts  in  preserving  a  tree  from  sudden  changes  of  tem- 
perature, enabling  it  to  resist  alike  a  sudden  abstraction  of  heat 
from  within,  and  the  sudden  accession  of  heat  from  without.  Tyndall 


Fig.  xE 


has  also  shown  that  this  tendency  is  aided  by  the  low  conducting 
power  of  the  bark,  which  is  in  all  cases  less  than  that  of  the  wood. 

Cotton,  wool,  straw,  bran,  powdered  gypsum,  etc.,  are  all  bad 
conductors. 

In  order  to  compare  the  conducting  power  or  conductivity  of  dif- 
ferent solids,  Ingenhousz  constructed  the  apparatus  which  bears  his 
rame,  and  which  is  represented  in  fig  188.  It  is  a  metal  trough,  in 
which,  by  means  of  tubulures  and  corks,  are  fixed  rods  of  the  same 
dimensions,  but  of  different  materials  ;  for  instance,  iron,  copper, 
wood,  glass.  These  rods  extend  to  a  slight  distance  in  the  trough, 
and  the  parts  outside  are  coated  with  wax,  which  melts  at  61°. 
The  box  being  filled  with  boiling  water,  it  is  observed  that  the 
wax  melts  to  a  certain  distance  on  the  metal  rods,  while  on.  the 


-219] 


Conducting  Power  of  Liqidds. 


21$ 


others  there  is  no  trace  of  fusion.  The  conducting  power  is  evidently 
greater  in  proportion  as  the  wax  has  fused  to  a  greater  distance. 
The  experiment  is  sometimes  modified  by  attaching  glass  balls  or 
marbles  to  the  ends  of  the  rods  by  means  of  wax.  As  the  wax 
melts,  the  balls  drop  off,  and  this  in  the  order  of  their  respective  con- 
ductivities. By  these  and  other  experiments  it  has  been  ascer- 
tained that  metals  are  the  best  conductors,  then  marble,  porcelain, 
brick,  wood,  glass,  etc. 

219.  Conducting  power  of  liquids.  Manner  in  which  they 
are  heated. — Liquids,  with  the  exception  of  mercury,  which  is  a 
metal,  are  all  bad  conductors  of  heat.  They  conduct  so  imperfectly 
that  Rumford  assumed  water  to  be  entirely  destitute  of  conducting 
power.  But  its  conductivity,  though  small,  has  been  established 
beyond  doubt,  as  well  as  that  of  other  liquids,  by  the  most  accurate 
experiments. 

From  their  small  conducting  power,  liquids  are  not  heated  in  the 
same  manner  as  solids.  If  heat  be  applied  to  a  solid,  whether  on 
the  top,  the  bottom,  or  the  sides,  it  is  transmitted  from  layer  to 
layer,  and  the  whole  mass  becomes  heated.  This  is  not  the  case 
with  a  liquid  ;  if  it  is  heated  at 
the  top,  the  heat  is  only  pro- 
pagated with  extreme  slowness, 
and  it  cannot  be  completely 
heated.  But  if  it  be  heated  at 
the  bottom,  the  temperature  of 
the  liquid  rapidly  rises.  This 
however,  is  not  owing  to  its 
conductivity,  but  to  ascending 
and  descending  currents,  which 
in  virtue  of  the  mobility  of  the 
molecules,  are  produced  through- 
out the  whole  mass  of  liquid. 

The  existence  of  these  cur- 
rents may  be  demonstrated  by 
placing  in  the  water  a  powder 
of  near  the  same  density,  for 
instance,  oak  sawdust,  and  then 
gently  heating  the  vessel  at  the  bottom.  As  the  lower  layers  of  the 
liquid  become  heated  they  expand,  while  the  upper  layers,  which  are 
colder  and  therefore  denser,  sink  and  take  the  place  of  the  first  ; 
these  in  their  turn  become  heated,  rise,  and  so  on,  until  the  entire 


Fig. 


216  On  Heat.  [219- 

mass  is  heated.  These  currents  are  evident  from  the  sawdust  which 
is  seen  to  rise  slowly  in  the  centre,  and  to  redescend  near  the  edges. 

220.  Conductivity  of  gases.— Gases  are  extremely  bad  con- 
ductors of  heat ;  but   this  cannot  be  easily  demonstrated  by  ex- 
periment, owing  to  the  extreme  mobility  of  their  particles.     For 
so  soon  as  they  are  heated  in  any  part  of  their   mass,  expan- 
sions and  currents  are  produced,  in  virtue  of  which  the  heated  parts 
mingle  with  the  cold  ones  ;  hence  a  general  elevation  of  tempera- 
ture, which  we  might  be  tempted  to  consider  as  due  to  conduc- 
tivity.    When,  however,  gases  are  hindered  in  their  motion,  their 
conductivity  seems  extremely  small,  as  the  following  examples  show. 

221.  Applications. — The  greater  or  less  conductivity  of  bodies 
meet  with  numerous  applications.     If  a  liquid  is  to  be  kept  warm 
for  a  long  time,  it  is  placed  in  a  vessel  and  packed  round  with  non- 
conducting substances,  such  as  shavings,  straw,  bruised  charcoal. 
For  this  purpose  water  pipes  and  pumps  are  wrapped  in  straw  at 
the  approach  of  frost.     The  same  means  are  used  to  hinder  a  body 
from  becoming  heated.     Ice  is  transported  in  summer  by  packing 
it  in  bran,  or  folding  it  in  flannel. 

Double  walls  constructed  of  thick  planks  having  between  them 
any  finely  divided  materials  such  as  shavings,  sawdust,  dry  leaves, 
etc.,  retain  heat  extremely  well ;  they  are  likewise  advantageous  in 
hot  countries,  for  they  prevent  its  access.  If  a  layer  of  asbestos, 
a  very  fibrous  substance,  is  placed  on  the  hand,  a  red-hot  iron  ball 
can  be  held  without  inconvenience.  Red-hot  cannon  balls  can  be 
wheeled  to  the  gun's  mouth  in  wooden  barrows  partially  filled  with 
sand.  Lava  has  been  known  to  flow  over  a  layer  of  ashes  under- 
neath which  was  a  bed  of  ice,  and  the  non-conducting  power  of  the 
ashes  has  prevented  the  ice  from  fusion.  A  covering  of  snow  pro- 
tects seed  and  young  grain  from  frost. 

In  fireproof  safes  the  hollow  walls  are  filled  with  wood  ashes, 
or  powdered  gypsum,  or  ignited  alum. 

The  clothes  which  we  wear  are  not  warm  in  themselves  ;  they 
only  hinder  the  body  from  losing  heat,  in  consequence  of  their 
spongy  texture  and  the  air  they  enclose.  The  warmth  of  bed-covers 
and  of  counterpanes  is  explained  in  a  similar  manner.  Double 
windows  are  frequently  used  in  cold  climates  to  keep  a  room  warm 
— they  do  this  by  the  non-conducting  layer  of  air  interposed  between 
them.  It  is  for  the  same  reason  that  two  shirts  are  warmer  than 
one  of  the  same  material,  but  of  double  the  thickness.  Hence  too 
the  warmth  of  furs,  eider  down,  etc. 


-222]  Expansion  of  Solids.  2 1 7 

That  water  boils  more  rapidly  in  a  metallic  vessel  than  in  one  of 
porcelain  of  the  same  thickness  ;  that  a  burning  piece  of  wood  can 
be  held  close  to  the  burning  part  with  the  naked  hand,  while  a 
piece  of  iron  heated  at  one  end  can  only  be  held  at  a  great  distance, 
are  easily  explained  by  reference  to  their  various  conductivities. 

The  sensation  of  heat  or  cold  which  we  feel  when  in  contact 
with  certain  bodies  is  materially  influenced  by  their  conductivity.  If 
their  temperature  is  lower  than  ours,  they  appear  colder  than  they 
really  are,  because,  from  their  conductivity,  heat  passes  away  from 
us.  If,  on  the  contrary,  their  temperature  is  higher  than  that  of  our 
body,  they  appear  warmer  from  the  heat  which  they  give  up  at  dif- 
ferent parts  of  their  mass.  Hence  it  is  clear  why  carpets,  for  ex- 
ample, are  warmer  than  wooden  floors,  and  why  the  latter  are 
warmer  than  stone  floors. 


CHAPTER  V. 

MEASUREMENT   OF    THE    EXPANSION    OF    SOLIDS,    LIQUIDS,    AND 

GASES. 

222.  Expansion  of  solids. — The  expansion  of  bodies  by  heat 
being  a  general  effect  which  exerts  its  influence  on  all  bodies,  and  is 
continually  changing  their  volume,  it  will  be  readily  understood  that 
the  determination  of  the  amount  of  this  expansion  is  a  problem  of 
great  importance,  both  in  its  purely  scientific,  as  well  as  in  its  prac- 
tical, aspects.  We  shall  first  describe  the  method  of  determining 
the  expansion  of  solids.  We  have  already  seen  that  the  expansion 
of  solids  may  be  either  as  regards  the  length  or  the  volume. 
Hence  the  investigation  of  the  expansion  of  solids  may  be  divided 
into  two  parts,  the  first  relating  to  linear,  and  the  second  to  cubical 
expansion. 

Linear  expansion.  In  order  to  compare  with  each  other  the  ex- 
pansion of  bodies,  the  elongation  is  taken  which  the  unit  of  length 
undergoes  when  he  is  heated  from  zero  to  i  degree,  and  this  elonga- 
tion is  called  the  coefficient  of  linear  expansion.  The  coefficients  ot 
a  great  number  of  substances  were  accurately  determined  towards 
the  end  of  the  last  century  by  Lavoisier  and  Laplace.  They  took 
a  bar  of  the  substance  to  be  determined,  placed  it  in  melting  ice, 
and  then  accurately  .determined  its  length.  Having  placed  it  then 
in  a  bath  of  boiling  water,  they  again  measured  its  length.  They 


218  On  Heat.  [222- 

then  observed  an  elongation,  which  represented  the  total  expansion 
for  an  increase  of  temperature  of  100  degrees.  This,  divided  by 
100,  gave  the  coefficient  of  linear  expansion  for  one  degree.  In  this 
manner  the  following  numbers  may  be  obtained  : — 

Coefficients  of  linear  expansion  for  i°  between  o°  and  100°  C. 
White  glass.  .  0-00000861  Bronze  .  .  0-000018167 
Platinum  .  .  0*00000884  Brass  .  .  .  0*000018782 
Steel  .  .  .  0-00001079  Silver .  .  .  0-000019097 
Iron  -.  .  .  0*00001220  Tin  .  .'  .  0-000021730 
Gold  .  »  0-00001466  Lead  .  .  0-000028575 

Copper          .         .     0*00001718         Zinc    .         .         .     0-000029417 

It  will  be  seen  from  this  table,  that  the  coefficients  of  expansion 
are  in  all  cases  very  small.  Thus,  when  we  say  that  the  coefficient 
of  expansion  of  copper  is  about  o-c 00017,  we  mean  that  a  rod  of  this 
metal  when  heated  through  i  degree,  will  expand  by  17  millionths 
of  its  length  ;  that  is  to  say,  a  rod  of  copper  a  million  feet  in  length 
would  be  longer  by  17  feet  under  these  circumstances. 

Cubical  expansion.  The  coefficient  of  cubical  expansion  is  the  in- 
crease in  volume  for  a  temperature  of  one  degree.  Calculation 
shows  that  the  coefficient  of  cubical  expansion  is  three  times  its 
coefficient  of  linear  expansion  ;  and  these  coefficients  may  therefore 
be  obtained  by  multiplying  the  above  numbers  by  three. 

223.  Applications  of  the  expansion  of  solids. — In  the  arts 
we  meet  with  numerous  examples  of  the  influence  of  expansion, 
(i.)  The  bars  of  furnaces  must  not  be  fitted  tightly  at  their  extrem- 
ities, but  must,  at  least,  be  free  at  one  end,  otherwise,  in  expanding, 
they  would  exert  sufficient  force  to  split  the  masonry,  (ii.)  In 
making  railways  a  small  space  is  left  between  the  successive  rails, 
for,  if  they  touched,  the  force  of  expansion  would  cause  them  to 
curve  or  would  break  the  chairs,  (iii.)  Water  pipes  are  fitted  to 
one  another  by  means  of  telescopic  joints,  which  allow  room  for 
expansion,  (iv.)  If  a  glass  is  heated  or  cooled  too  rapidly  it 
cracks  ;  this  arises  from  the  fact  that  glass  being  a  bad  conductor 
of  heat,  the  sides  become  unequally  heated,  and  consequently  un- 
equally expanded,  and  the  strain  thereby  produced  is  sufficient  to 
cause  a  fracture. 

When  bodies  have  been  heated  to  a  high  temperature,  the  force 
produced  by  their  contraction  on  cooling  is  very  considerable  ;  it 
is  equal  to  the  force  which  is  needed  to  compress  or  expand  the 
material  to  the  same  extent  by  mechanical  means.  According  to 


—224]  Compensation  Pendulum.  219 

Barlow  a  bar  of  malleable  iron  a  square  inch  in  section  is  stretched 
I6£--  of  its  length  by  a  weight  of  a  ton  ;  the  same  increase  is  ex- 
perienced by  about  9°  C.  A  difference  of  45°  C.  between  the  cold 
of  winter  and  the  heat  of  summer  is  not  unfrequently  experienced 
in  this  country.  In  that  range  a  wrought  iron  bar,  ten  inches  long 
will  vary  in  length  by  ~o  of  an  inch,  and  will  exert  a  strain,  if  its 
ends  are  securely  fastened,  of  fifty  tons. 

An  application  of  this  contractile  force  is  seen  in  the  mode  of 
securing  the  tires  on  wheels.  The  tire  being  made  red  hot,  and 
thus  considerably  expanded,  is  placed  on  the  circumference  of  the 
wheel,  and  then  cooled.  The  tire,  when  cold,  clasps  the  wheel 
with  such  force  as  not  only  to  secure  itself  on  the  rim,  but  also  to 
press  home  the  joints  of  the  spokes  into  the  felloes  and  nave. 
Another  interesting  application  was  made  in  the  case  of  a  gallery 
at  the  Conservatoire  des  Arts  et  Metiers  in  Paris,  the  walls  of  which 
had  begun  to  bulge  outwards.  Iron  bars  were  passed  across  the 
building,  and  screwed  into  plates  on  the  outside  of  the  walls.  Each 
alternate  bar  was  then  heated  by  means  of  lamps,  and  when  the 
bar  had  expanded,  it  was  screwed  up.  The  bars  being  then  allowed 
to  cool  contracted,  and  in  so  doing  drew  the  walls  together.  The 
same  operation  was  performed  on  the  other  bars. 

224.  Compensation  pendulum. — An  important  application  of 
the  expansion  of  metals  has  been  made  in  the  compensation  pendu- 
lum. To  understand  the  utility  of  such  an  arrangement,  we  must 
call  to  mind  what  has  been  said  about  pendulums  ;  namely,  that 
their  oscillations  are  isochronous,  that  is,  are  made  in  equal  times, 
and  that  their  application  to  the  regulation  of  clocks  depends  upon 
this  property.  But  we  have  also  seen  that  the  duration  of  an  oscil- 
lation depends  on  the  length  of  the  pendulum  ;  the  longer  the  pen- 
dulum the  more  slowly  it  oscillates,  and,  therefore,  the  shorter  it  is, 
the  more  rapidly  does  it  oscillate.  Hence  a  pendulum  formed  of  a 
single  rod  terminated  by  a  metal  bob,  c,  as  represented  in  fig.  52, 
could  not  be  an  exact  regulator  ;  for,  as  the  temperature  rises,  it 
would  elongate,  and  the  clock  would  go  slower  :  the  exact  opposite 
would  take  place  when  it  contracted  by  cooling.  These  inconve- 
niences have  been  remedied  by  taking  the  remedy  from  the  cause 
of  the  evil. 

For  this  purpose  the  pendulum  rod  consists  of  several  metal 
bars  arranged  as  represented  in  fig.  190.  The  rods,  a,b,  c,  d,  are  of 
steel,  and  all  expand  in  a  downward  direction  when  the  tempera- 
ture rises,  thus  making  the  bob  sink.  The  rod,  d,  supporting  the 


220 


On  Heat. 


[224- 


bob  is  fixed  to  a  cross-piece  mn,  which  in  turn  is  fastened  to  two 
rods,  k  and  /i,  which  are  connected  to  the  piece  or,  and  therefore 
cannot  expand  downwards,  but  only  in  an  upward  direction ;  they 
raise  the  piece  ;//«,  and  with  it  the  bob.  In  order,  therefore,  that 
this  latter  shall  neither  raise  nor  sink,  it  is  necessary  that  the  upward 
expansion  of  the  rods,  k  and  h,  shall  exactly  com- 
pensate the  downward  expansion  of  the  rods,tf,£,  c,d. 
Brass  being  more  expansible  than  steel,  com- 
pensation is  effected  by  taking  the  first  metal  for 
the  rods  h  and  k,  and  the  second  for  the  rods,  a,  b,  r, 
and  d.  The  only  condition  necessary  for  com- 
pensation is  that  the  lengths  of  the  two  metals  must 
be  inversely  as  their  coefficients  of  expansion.  That 
is  to  say,  that  if  brass  is  two  or  three  times  as  ex- 
pansible as  steel,  its  length  must  be  one- half  or  one- 
third  as  much. 

In  fig.  190  the  pendulum  has  been  represented 
with  a  single  frame  of  steel  and  one  of  brass  ;  but 
in  order  to  reduce  the  length,  there  are  always  at 
least  two  rows  of  steel  and  brass. 

EXPANSION   OF   LIQUIDS. 


225.  Absolute    and    apparent    expansions. — 

We  have  already  seen  that  liquids  are  more  expan- 
sible than  solids  (200),  which  is  a  consequence  of 
their  feeble  cohesion  ;  but  their  expansibility  is  far 
less  regular,  and  the  less  so  the  nearer  their  tempera- 
ture approaches  that  of  their  boiling  point. 

In  solids  two  kinds  of  expansion  have  to  be  con- 
sidered, the  longitudinal  and  the  cubical.     Now  it 
is  clear  that  the  latter  is  the  only  kind  of  expansion 
Fig.  190.          which    can  be   observed  in   the   case   of  liquids. 
The   expansion  may  be  either  real  or  apparent. 
The  former  is  the  real  increase  in  volume  which  a  liquid  assumes 
when  it  is  heated ;  while  the  latter  is  that  which  the  eye  actually 
observes,  that  produced  in  the  vessel  containing  the  liquid.     Thus 
in  thermometers,  when  the  liquid  expands  and  rises  in  the  stem,  the 
apparent  expansion  is  observed,  which  is  less  than  the  real  or  abso- 
lute expansion.     For,  while  the  mercury  expands,  the  bulb  of  the 
thermometer  does  so  too  ;    its  volume  is  greater,  and  hence  the 


-226] 


Maximum  Density  of  Water. 


22\ 


liquid  does  not  rise  so  high  in  the  stem  as  it  would  if  the  volume 
of  the  bulb  were  unaltered.  If  a  flask  of  thin  glass,  provided  with 
a  capillary  stem,  the  flask  and  part  of  the  stem  being  filled  with 
some  coloured  liquid,  be  immersed  in  hot  water,  the  column  of 
liquid  in  the  stem  at  first  sinks,  but  then  immediately  after  rises, 
and  continues  to  do  so  until  the  liquid  inside  has  the  same 
temperature  as  the  hot  water.  The  first  sinking  of  the  liquid  is  not 
due  to  its  contraction  ;  it  arises  from  the  expansion  of  the  glass, 
which  becomes  heated  before  the  heat  can  reach  the  liquid  ;  but 
the  expansion  of  the  liquid  soon  exceeds  that  of  the  glass,  and  the 
liquid  then  ascends. 

Hence,  since,  whatever  be  the  nature  of  the  material  in  which  a 
liquid  is  contained,  it  has  some  expansibility,  and  always  expands 
with,  the  liquid,  the  apparent  expansion  is  the  only  one  directly  ob- 
served in  liquids. 

The  coefficient  of  expansion  of  a  liquid  is  the  increase  which 
the  unit  of  volume  experiences  for  a  rise  in  temperature  of  one 
degree.  These  coefficients  greatly  vary.  In  a  glass  vessel  the 
apparent  expansion  of  mercury  is  1-5  parts  in  ten  thousands  ;  that 

of  water  is  4-6  parts,  that  is,  three  times  as  great  ;  alcohol  is  still 

more  expansible,  for  its  coefficient  is  ir6  parts  in  ten  thousands. 
226.   Maximum   density  of 

•water. — Water  presents  the  re- 
markable phenomenon  that  when 

its  temperature  sinks  it  contracts 

up  to  4°  ;   but  from  that  point, 

although  the  cooling  continues, 

it  expands   up   to   the  freezing 

point,  so  that  4°  represent  the 

point  of  greatest  contraction  of 

water,  or,  what  is  called,  its  point 

of  maximum  density, 

These  phenomenon  may  be 

observed  by  comparing  a  water 

thermometer,  one,  that  is  to  say. 

filled   with  water,  with   one    of 

mercury  ;    both   being   exposed 

to  gradually  diminishing  tempe- 
rature. 

Hope     used    the    following 

method  to  determine  the  maximum   density  of  water.    He  took 


222  On  Heat.  [226- 

a  deep  vessel  perforated  by  two  lateral  apertures,  in  which  he  fixed 
thermometers  (fig.  190),  and  having  filled  the  vessel  with  water  at  o°, 
he  placed  it  in  a  room  at  a  temperature  of  1 5°.  As  the  layers  of 
liquid  at  the  sides  of  the  vessel  became  heated  they  sank  to  the 
bottom,  and  the  lower  thermometer  marked  4°,  while  that  of  the 
upper  one  was  still  at  zero.  Hope  then  made  the  inverse  experi- 
ment ;  having  filled  the  vessel  with  water  at  15°,  he  placed  it  in  a 
room  at  zero.  The  lower  thermometer  having  sunk  to  4°,  remained 
stationary  for  some  time,  while  the  upper  one  cooled  down  until  it 
reached  zero.  Both  these  experiments  prove  that  water  is  heavier 
at  4°  than  at  o°,  for  in  both  cases  it  sinks  to  the  lower  part  of  the 
vessel. 

This  phenomenon  is  of  great  importance  in  the  economy  of 
nature.  In  winter  the  temperature  of  lakes  and  rivers  falls,  from 
being  in  contact  with  the  cold  air,  and  from  other  causes,  such  as 
radiation.  The  colder  water  sinks  to  the  bottom,  and  a  continual 
series  of  currents  goes  on  until  the  whole  has  a  temperature  of  4°. 
The  cooling  on  the  surface  still  continues,  but  the  cooled  layers 
being  lighter  remain  on  the  surface,  and  ultimately  freeze.  The  ice 
formed  thus  protects  the  water  below,  which  remains  at  a  tempera- 
ture of  4°,  even  in  the  most  severe  winters,  a  temperature  at  which 
fish  and  other  inhabitants  of  the  waters  are  not  destroyed. 


EXPANSION   OF  GASES. 

227.  Value  of  the  coefficient  of  expansion  of  gases. — Net 

merely  are  gases  the  most  expansible  of  all  bodies,  but  their  expan- 
sion is  the  most  regular.  It  was  originally  assumed,  on  the  basis 
of  Gay  Lussac's  experiments,  that  all  gases  expanded  to  the  same 
extent  for  the  same  increase  of  temperature,  that  is,  that  they  had 
all  the  same  coefficient  of  expansion.  It  has,  however,  been 
established  that  the  coefficients  of  various  gases  do  present  slight 
differences.  They  are,  however,  so  slight,  that  for  all  practical 
purposes  they  may  be  assumed  to  be  the  same ;  that  is  to  say,  367 
parts  in  a  hundred  thousand,  or,  in  other  words,  that  100,000 
volumes  of  air,  or  any  other  gas,  when  heated  through  i  degree 
Centigrade,  would  become  100,367  volumes  or  i  volume  in  273. 
This  expansibility  is  about  13  times  as  great  as  that  of  water. 

228.  Effects  of  the  expansion   of  gases. — The   expansion  of 
gases  affords  us  numerous  important  applications,  not  merely  in 


-228]          Effects  of  the  Expansion  of  Gases.  223 

domestic  economy,  but  also  in  atmospheric  phenomena.  Thus  in 
our  dwellings,  when  the  air  is  heated  and  vitiated  by  the  presence 
of  a  great  number  of  persons,  it  expands  and  rises  in  virtue  of  its 
diminished  density  to  the  highest  parts  of  rooms  ;  and  to  allow  this 
to  escape,  apertures  are  made  in  the  cornice,  while  fresh  and  pure 
air  enters  by  the  joints  of  the  doors  and  of  the  windows. 

When  in  winter  the  door  of  a  warm  room  is  put  ajar,  and  a 
lighted  candle  held  near  the  top,  fig.  192,  the  direction  of  the  flame 
proves  the  existence  of  a  current  of  warm 
air  from  the  inside  to  the  outside.  If  we 
lower  the  flame,  it  will  be  found  that  at 
about  the  middle  it  is  not  affected  by  any 
air  current,  but  that  lower  down  near  the 
ground,  the  flame  is  driven  inwards. 

In  theatres  the  spectators  in  the  gal- 
leries are  exposed  to  the  highest  tempera- 
ture and  the  most  impure  air,  while  those 
near  the  orchestra  respire  in  a  purer  air. 

Draughts  in  chimneys  are  due  to  the 
expansion  of  air.  Heated  by  the  fire  in 
the  grate,  the  air  rises  in  the  chimney 

with  a  velocity  which  is  greater  the  more  it  is  expanded.  Hence 
results  a  rapid  current  of  air,  which  supports  and  quickens  the 
combustion  by  constantly  renewing  the  oxygen  absorbed. 

The  expansion  and  contraction  of  air  have  a  fortunate  influence 
on  the  temperature  of  that  part  of  the  atmosphere  in  which  we 
live.  For  when  the  ground  is  strongly  heated  by  the  sun's  burning 
rays,  the  layers  of  air  in  immediate  contact  with  it  tend  to  acquire 
the  same  temperature  and  to  form  a  stifling  atmosphere  ;  but  these 
layers,  gradually  expanding,  rise  in  virtue  of  their  diminished  den- 
sity ;  while  the  higher  layers,  which  are  colder  and  denser,  gradually 
replace  them.  Thus  the  high  temperature  whidi  would  otherwise 
be  produced  in  the  lower  regions  is  moderated,  and  never  exceeds 
the  limits  which  plants  and  animals  can  support. 

The  expansion  and  contraction  produced  in  the  atmosphere  over 
a  large  tract  of  country  are  the  cause  of  all  winds,  from  the  lightest 
zephyr  to  the  most  violent  hurricane.  These  winds,  which  at 
times  are  so  destructive,  so  capricious  in  the  direction,  and  so 
variable  in  their  intensity,  not  merely  have  the  effect  of  mixing  the 
heated  and  the  cooler  part  of  the  atmosphere,  and  of  thus  modera- 
ting extremes  of  temperature,  but  by  driving  away  the  vitiated  at- 


224  On  Heat.  [228- 

mosphere  of  our  towns,  and  replacing  it  by  pure  air,  they  are  one 
of  the  principal  causes  of  salubrity  ;  without  them  our  cities  would 
be  the  centres  of  infection,  where  epidemic  diseases  of  all  kinds 
would  be  permanent.  Without  winds,  clouds  would  remain  motion- 
less over  the  country  where  they  were  formed,  the  greater  part  of 
the  globe  would  be  condemned  to  absolute  aridity,  and  neither 
rivers  nor  brooks  would  moisten  the  soil.  But,  carried  by  the 
winds,  the  clouds  formed  above  the  seas  are  transported  to  the 
centres  of  continents,  where  they  fall  as  rain  ;  and  this  having 
fertilised  the  soil,  gives  rise  to  the  numerous  rivers  which  fall  into 
the  ocean,  thus  establishing  a  continuous  circulation  from  the  seas 
towards  the  continents  and  from  continents  towards  seas. 

229.  Density  of  gases.— The  densities  of  solids  and  of  liquids 
have  been  determined  in  reference  to  water  (101)  ;  those  of  gases 
by  comparison  with  air  ;  that  is,  having  taken:  as  a  term  of  com- 
parison, or  unity,  the  weight  of  a  certain  volume  .of  air,  the  weight 
of  the  same  volume  of  other  gases  is  determined.  But  as  gases 
are  very  compressible  and  very  expansible,  and'as  therefore  their 
densities  may  greatly  vary,  they  must  be  reduced  to  a  definite  pres- 
sure and  temperature.  This  is  why  the  temperature  of  zero  and  the 
pressure  of  30  inches  have  been  chosen.  ;  •;.'.;  i. 

Hence  the  relative  density  of  a  gas,  ,or  its  specific  gravity,  is  the 
relation  of  the  weight  of  a  certain  volume  of  the  gas  to  that  of  the 
same  volume  of  air  ;  both  the  gas  and  the  air  being  at  zero  and  at 
a  pressure  of  30  inches. 

In  order  therefore,  to  find  the  specific  gravity  of  a  gas,  oxygen 
for  instance,  it  is  necessary  to  determine  the  weight  of  a  certain 
volume  of  this  gas,  at  a  pressure  of  30  inches,  and  a  temperature  of 
zero,  and  then  the  weight  of  the  same  volume  of  air  under  the  same 
conditions.  For  this  purpose  a  large  globe  of  about  two  gallons 
capacity  is  used,  like  that  represented  in  fig.  88,  the  neck  of  which 
is  provided  with  a  stop  cock,  which  can  be  screwed  to  the  air-pump. 
The  globe  is  first  weighed  empty,  and  then  full  of  air,  and  after- 
wards full  of  the  gas  in  question.  The  weights  of  the  gas  and  of 
the  air  are  obtained  by  substracting  the  weight  of  the  exhausted 
globe  from  the  weight  of  the  globes  filled,  respectively,  with  air 
and  gas.  The  quotient,  obtained  by  dividing  the  latter  by  the 
former,  gives  the  specific  gravity  of  the  gas.  It  is  difficult  to  make 
these  determinations  at  the  same  temperature  and  pressure,  and 
therefore  all  the  weights  are  reduced  by  calculation  to  zero,  and  the 
standard  pressure  of  30  inches. 


-230]  Fusion.  22$ 

In  this  manner  the  following  densities  have  been  found  : 

Air  ....  I'oooo  Oxygen  V  ;  .  .  1*1056 
Hydrogen  .  .  .  0-0692  Carbonic  acid  .  _  .'"''•'  1-5290 
Nitrogen  .  .  .  0*9714  Chlorine  ..  ^  ......  .  3-4400 

From  these   numbers   the  lightest  of  gases,  and  therefore  of  a 
bodies,  is  hydrogen,  whose  density  is  less  than  T\th  of  air. 


CHAPTER  VI.     • 

CHANGES   OF   STATE  OF   BODIES    BY  THE  ACTION   OF   HEAT. 

230.  Fusion.  —  In  treating  of  the  general  effects  of  heat,  we  have 
seen  that  its  action  is  not  only  to  expand  them,  but  to  cause  them 
to  pass  from  the  solid  to  the  liquid  state,  or  from  the  latter  state 
to  the  former,  according  as  the  temperature  rises  or  falls  ;  then 
from  the  liquid  to  the  aeriform  state,  or  conversely.  These  vari- 
ous changes  of  slate  we  shall  now  investigate  under  the  name  of 
fusion,  solidification,  vaporisation,  and  liquefaction. 

Fusion  is  the  .passage  of  a  solid  body  to  the  liquid  state  by  the 
action  of  heat.  This  phenomenon  is  produced  when  the  force  of 
cohesion  which  unites  the  molecules  is  balanced  by  the  force  of  re- 
pulsion (4)  ;  but  as  the  cohesive  force  varies  in  different  substances, 
the  temperature  at  which  bodies  melt  does  so  likewise.  For  some 
substances  this  temperature  is  very  low,  and  for  others  very  high,  as 
the  following  table  shows  : 


Fusing  points  of  certain  substances. 


Mercury. 

.-38-8° 

Sulphur 

Bromine  . 

Uv       .     12-5 

Tin      '  . 

Ice. 

Bismuth 

Butter     . 

•  +  33 

Lead      . 

Phosphorus     . 

•    44 

Zinc 

Potassium 

•     55 

Antimony 

Stearine  . 

.    60 

Silver     . 

White  wax 

.        .    65 

Gold      . 

Sodium  . 

•    90 

Iron 

Q 

228 
264 

335 
422 
450 

looo 

1250 
1  500 


226  On  Heat.  [230- 

Some  substances,  however,  such  as  paper,  wood,  wool,  and 
certain  salts,  do  not  fuse  at  a  high  temperature,  but  are  decomposed. 
Many  bodies  have  long  been  considered  refractory ;  that  is,  in- 
capable of  fusion  ;  but,  in  the  degree  in  which  it  has  been  possible 
to  produce  higher  temperatures,  their  number  has  diminished. 
Gaudin  has  succeeded  in  fusing  rock  crystal  by  means  of  a  lamp 
fed  by  a  jet  of  oxygen  ;  and  more  recently  Despretz,  by  combining 
the  effects  of  the  sun,  the  voltaic  battery,  and  the  oxy-hydrogen  blow- 
pipe, has  melted  alumina  and  magnesia,  and  has  softened  carbon, 
so  that  it  was  flexible,  which  is  a  condition  near  that  of  fusion. 

Some  substances  pass  from  the  solid  to  the  liquid  state  without 
showing  any  definite  melting  point  ;  for  example,  glass  and  iron  be- 
come gradually  softer  and  softer  when  heated,  and  pass  by  imper- 
ceptible stages  from  the  solid  to  the  liquid  condition.  This  inter- 
mediate condition  is  spoken  of  as  the  state  of  vitreous  ftision. 
Such  substances  may  be  said  to  melt  at  the  lowest  temperature 
at  which  perceptible  softening  occurs,  and  to  be  fully  melted  when 
the  further  elevation  of  temperature  does  not  make  them  more 
fluid  ;  but  no  precise  temperatures  can  be  given  as  their  melting 
points. 

231.  Xiaws  of  fusion. — It  has  been  experimentally  found,  that 
the  fusion  of  bodies  is  governed  by  the  two  following  laws  : 

I.  Every  substance  begins  to  fuse   at  a  certain   temperature, 
which  is  invariable  for  one  and  the  same  substance  if  the  pressure  be 
constant. 

II.  Whatever  be  the  intensity  of  the  source  of  heat,  from  the  mo~ 
ment  fusion  commences,  the  temperature  of  the  body  ceases  to  rise,  and 
remains  constant  until  the  fusion  is  complete. 

For  instance,  the  melting  point  of  ice  is  zero,  and  a  piece  of  this 
substance  exposed  to  the  sun's  rays,  placed  in  front  of  a  fire  or 
over  a  lamp,  could  never  be  heated  beyond  this  temperature.  Ex- 
posure to  a  more  intense  heat  would  only  accelerate  the  fusion,  the 
temperature  would  remain  at  zero  until  the  whole  of  the  ice  was 
melted. 

232.  latent  heat. — Since,  during  the  passage  of  a  body  from 
the  solid  to  the  liquid  state,  the  temperature  remains  constant  until 
the  fusion  is  complete,  whatever  be  the  intensity  of  the  source  of 
heat,  it  must  be  concluded  that,  in  changing  their  condition,  bodies 
absorb  a  considerable  amount  of  heat,  the  only  effect  of  which  is  to 
maintain  them  in  the  liquid  state.     This  heat,  which  is  not  indicated 
by  the  thermometer,  is  called  latent  heat,  or  latent  heat  of  fusion 


-233]  Solidification.  227 

an  expression  which,  though  not  in  strict  accordance  with  modern 
ideas,  is  convenient  from  the  fact  of  its  universal  recognition  and 
employment. 

An  idea  of  what  is  meant  by  latent  heat  may  be  obtained  from 
the  following  experiment.  If  a  pound  of  water  at  80°' is  mixed  with 
a  pound  of  water  at  zero,  the  temperature  of  the  mixture  is  40°. 
But  if  a  pound  of  pounded  ice  at  zero  is  mixed  with  a  pound  of 
water  at  80°,  the  ice  melts,  and  two  pounds  of  water  at  zero  are 
obtained.  The  pound  of  ice  at  zero  is  changed  into  a  pound  of 
water  also  at  zero,  but  as  the  hot  water  is.  also  lowered  to  this  tem- 
perature, what  has  become  of  the  80°  of  heat  it  possessed  ?  They 
exist  in  the  water  which  results  from  the  ice  ;  their  effect  has 
neither  been  to  increase  its  temperature  nor  its  volume,  but  simply 
to  impart  fluidity  to  it.  Consequently,  the  mere  change  of  a  pound 
of  ice  to  a  pound  of  water  at  the  same  temperature  requires  as  much 
heat  as  will  raise  a  pound  of  water  through  80°.  This  quantity  of 
heat  represents  the  latent  heat  of  the  fusion  of  ice,  or  the  latent 
heat  of  water. 

Every  substance  in  melting  absorbs  a  certain  amount  of  heat, 
which,  however,  varies  materially  with  different  substances. 

The  enormous  quantity  of  heat  absorbed  by  ice  in  melting,  ex- 
plains how  it  is  that  so  long  a  time  is  required  for  thaw.  And 
conversely,  it  is  owing  to  the  latent  heat  of  water,  that  even  when 
its  temperature  has  been  reduced  to  zero,  so  long  a  time  is  required 
before  it  is  entirely  frozen.  Before  it  can  be  so  it  must  give  out 
the  heat  which  had  been  consumed  in  its  liquefaction  :  it  thus  be- 
comes a  source  of  heat  which  retards  the  solidification.  Faraday 
has  calculated  that  the  heat  given  out  by  a  cubic  yard  of  water  in 
freezing  is  equal  to  that  which  would  be  produced  by  the  complete 
combustion  of  a  bushel  of  coals. 

Were  it  not  for  the  great  amount  of  heat  which  must  be  absor- 
bed by  snow  or  ice  in  melting,  we  should,  on  a  change  from  frost  to 
mild  weather,  be  liable  to  the  most  destructive  floods,  from  the  sud- 
den melting  of  the  accumulated  snow  and  ice. 

233.  Solidification. — Those  substances  which  are  liquefied  by 
heat  revert  to  the  solid  state  on  cooling,  and  this  passage  from  the 
liquid  to  the  solid  state  is  called  solidification.  If  this  solidification 
takes  place  at  a  low  temperature  it  is  frequently  spoken  of  as  con- 
gelation. 

In  all  cases  the  phenomenon  is  subject  to  the  following  laws  : 

Q  2 


223  On  Heat.  [233- 

I.  Every  body,  under  the  same  pressure,  solidifies  at  a  fixed  tem- 
perature, which  is  the  same  as  that  of  fusion. 

II.  From  the  commencement  to  the  end  of  the  solidification,  the 
temperature  of  a  liquid  remains  constant. 

Thus  if  lead  begins  to  melt  at  335°,  melted  lead  in  like  manner 
when  cooled  down  begins  to  solidify  at  335°.  Moreover,  until  it  is 
completely  solidified,  the  temperature  remains  constant  at  335°. 
This  arises  from  the  fact,  that  the  liquid  metal  in  proportion  as  it 
solidifies  restores  the  heat  it  had  absorbed  in  being  melted.  The 
same  phenomenon  is  observed  whenever  a  liquid  solidifies  (^32). 

Many  liquids,  such  as  alcohol,  ether,  and  bisulphide  of  carbon, 
do  not  solidify  even  at  the  lowest  known  temperature.  Pure  water 
solidifies  at  zero;  salt  water  at  — 2-5°,  olive  and  rape  oils  at  —  6°  ; 
linseed  and  nut  oils  at  —27°. 

Water  presents  the  remarkable  phenomenon,  that  when  it  solidi- 
fies and  forms  ice  its  volume  undergoes  a  material  increase.  In 
speaking  of  the  maximum  density  of  water  we  have  already  seen 
that,  on  cooling,  it  expands  from  4  degrees  to  zero  ;  it  further  ex- 
pands on  the  moment  of  solidifying,  or  contracts  on  melting  by 
about  10  per  cent.  One  volume  of  ice  at  o°  gives  0-908  of  water  at 
o°,  or  i  volume  of  water  at  o°  gives  1-102  of  ice  at  the  same  tem- 
perature. 

The  increase  of  volume  in  the  formation  of  ice  is  accompanied 
by  an  expansive  force  which  sometimes  produces  powerful  mecha- 
nical effects,  of  which  the  bursting  of  water  pipes  and  the  breaking 
of  jugs  containing  water  are  familiar  examples.  The  splitting  ot 
stones,  rocks,  and  the  swelling  up  of  moist  ground  during  frost,  are 
caused  by  the  fact  that  water  penetrates  into  the  pores  and  there 
becomes  frozen. 

The  expansive  force  of  ice  was  strikingly  shown  by  some  experi- 
ments of  Major  Williams  in  Canada.  Having 
quite  filled  a  1 3-inch  iron  bomb-shell  with 
water,  he  firmly  closed  the  touch-hole  with  an 
iron  plug  weighing  3  pounds,  and  exposed  it 
in  this  state  to  the  frost.  After  some  time 
the  iron  plug  was  forced  out  with  a  loud  ex- 
Fig-  193-  plosion,  and  thrown  to  a  distance  of  415  feet, 
the  shell  was  cracked,  and  a  mass  of  ice  projected  from  the  crack  as 
shown  in  fig.  193. 

From  the  expansion  which  water  undergoes  in  freezing,  it  is 
clear  that  ice  must  be  less  dense  than  water ;  and  this  in  fact  is  the 


-235]  Solution.  229 

case,  for  ice  floats  on  the  surface  of  the  water.  In  the  polar  seas, 
where  the  temperature  is  always  very  low,  masses  of  floating  ice  are 
met  with  which  are  called  ice-fields.  They  rise  out  of  the  sea  to  a 
height  of  4  or  5  yards,  and  are  immersed  to  a  depth  of  7  or  8  yards, 
and  they  frequently  extend  over  40  miles.  True  mountains  of  ice, 
or  icebergs,  are  found  floating  on  those  seas ;  they  have  not  the 
same  area,  but  attain  very  great  heights. 

Cast-iron,  bismuth,  and  antimony  expand,  on  solidifying  like 
water,  and  can  thus  be  used  for  casting  ;  but  gold,  silver,  and  cop- 
per contract,  and  hence  coins  of  these  metals  cannot  be  cast,  but 
must  be  stamped  with  a  die. 

234.  Crystallisation. — When  bodies  pass  slowly  from  the  liquid 
to  the  solid  state,  their  molecules,  instead  of  becoming  grouped  in 
a  confused  manner,  generally  acquire  a  regular  order  and  arrange- 
ment, in  virtue  of  which  these  bodies  assume  the  geometrical  shapes 
of  cubes,  pyramids,  and  prisms,  etc.,  which  are  perfectly  definite, 
and  are  known  as  crystals.     Flakes  of  snow,  when  looked  at  under 
the  microscope,  ice  in  the  process  of  formation,  sugar  candy,  rock 
crystal,  alum,  common  salt,  and  many  other  substances  afford  well- 
known  instances  of  crystallisation. 

Two  methods  are  in  use  for  crystallising  substances  ;  the  dry 
way  and  the  moist  way.  By  the  first  method  bodies  are  melted  by 
heat,  and  then  allowed  to  cool  slowly.  The  vessel  in  which  the 
operation  is  performed  becomes  lined  with  crystals,  which  are  made 
apparent  by  inverting  the  vessel  and  pouring  out  the  excess  of 
liquid  befoVe  the  whole  of"  it  is  melted.  Sulphur,  bismuth,  and 
many  other  metals  are  thus  easily  crystallised.  The  second  method 
consists  in  dissolving  in  hot  water  the  substance  to  be  crystallised, 
and  then  allowing  it  to  cool  slowly.  The  body  is  then  deposited  on 
the  sides  of  vessels  in  crystals  which  are  larger  and  better  shaped 
the  more  slowly  the  crystallisation  is  effected.  In  this  manner 
sugar  candy  and  salts  are  crystallised. 

235.  Solution. — A  body  is  said  to  dissolve  when  it  becomes 
liquid  in  consequence  of  an  affinity  between   its   molecules  and 
those  of  a  liquid.     Gum  arabic,  sugar,  and  most  salts  dissolve  in 
water. 

During  solution,  as  well  as  during  fusion,  a  certain  quantity  of 
heat  always  becomes  latent,  and  hence  it  is  that  the  solution  of  a 
substance  usually  produces  a  diminution  of  temperature.  In  cer- 
tain cases,  however,  instead  of  the  temperature  being  lowered,  it 
actually  rises,  as  when  caustic  potass  is  dissolved  in  water,  This 


230  On  Heat.  [235- 

depends  upon  the  fact  that  during  the  solution  ofa  solid  in  a  liquid, 
two  simultaneous  and  contrary  phenomena  are  produced.  The  first 
is  the  passage  from  the  solid  to  the  liquid  condition,  which  always 
lowers  the  temperature.  The  second  is  the  chemical  combination 
of  the  body  dissolved  with  the  liquid,  and  which,  as  in  the  case  of 
all  chemical  combinations,  produces  an  increase  of  temperature. 
Consequently,  as  the  one  or  the  other  of  these  effects  predominates, 
or  as  they  are  equal,  the  temperature  either  rises,  or  sinks,  or  re- 
mains constant. 

236.  Freezing-  mixtures. — The  absorption  of  heat  in  the  pas- 
sage of  bodies  from  the  solid  to  the  liquid  state  has  been  used  to 
produce  artificial  cold.  This  is  effected  by  mixing  together  bodies 
which  have  an  affinity  for  each  other,  and  of  which  one  at  least  is 
solid,  such  as  water  and  a  salt,  ice  and  a  salt,  or  an  acid  and  a  salt. 
Chemical  affinity  accelerates  the  fusion,  the  portion  which  melts 
robs  the  rest  of  the  mixture  of  a  large  quantity  of  sensible  heat, 
which  thus  becomes  latent.  In  many  cases  a  very  considerable 
diminution  of  temperature  is  produced. 

If  the  substances  taken  be  themselves  first  previously  cooled 
down,  a  still  more  considerable  diminution  of  temperature  is  occa- 
sioned. 

Freezing  mixtures  are  frequently  used  in  chemistry,  in  physics, 
and  in  domestic  economy.  The  portable  ice-making  machines 
which  have  come  into  use  during  the  last  few  years,  consist  of  a 
cylindrical  metallic  vessel  divided  into  four  concentric  compart- 
ments. In  the  central  one  is  placed  the  water  to  be  fro'zen  ;  in  the 
next  there  is  the  freezing  mixture,  which  usually  consists  of  sul- 
phate of  sodium  and  hydrochloric  acid ;  6  pounds  of  the  former 
and  5  of  the  latter  will  make  5  to  6  pounds  of  ice  in  an  hour.  The 
third  compartment  also  contains  water,  and  the  outside  one  con- 
tains some  badly  conducting  substance,  such  as  cotton,  to  prevent 
the  influence  of  the  external  temperature.  The  best  effect  is 
obtained  when  pretty  large  quantities,  2  or  3  pounds,  of  the  mix- 
ture are  used,  and  when  they  are  intimately  mixed.  It  is  also 
advantageous  to  use  the  machines  for  a  series  of  successive  opera- 
tions. 


-238]  Elastic  Force  of  Vapours.  231 


CHAPTER  VII. 

FORMATION   OF  VAPOURS.      MEASUREMENT  OF  THEIR  ELASTIC 

FORCE. 

237.  Vapours.— We  have  already  seen  (109)  that  vapours  are 
the  aeriform  fluids  into  which  substances,  such  as  ether,  alcohol, 
water,  and  mercury,  are  changed  by  the  absorption  of  heat. 

In  respect  to  the  property  of  disengaging  vapours,  liquids  are 
divided  into  two  classes,  volatile  liquids,  and  fixed  liquids.  The 
first  are  those  which  have  a  tendency  to  pass  into  the  state  of 
vapour  at  the  ordinary  or  even  at  lower  temperatures ;  such,  for 
instance,  are  water,  ether,  chloroform,  alcohol,  which  disappear  more 
or  less  rapidly  when  exposed  to  the  air  in  open  vessels.  To  this  class 
belongs  a  numerous  family  of  liquids  met  with  in  nature,  such  as 
essence  of  turpentine,  oil  of  lemons,  of  lavender,  of  thyme,  of  roses, 
etc.,  which  are  known  as  the  essential  oils. 

Fixed  liquids,  on  the  contrary,  are  those  which  emit  no  vapour 
at  any  temperature ;  such,  for  instance,  are  the  fatty  oils,  as  olive, 
rape,  etc.  When  strongly  heated  these  oils  are  decomposed,  and 
give  rise  to  gaseous  products  ;  but  they  do  not  emit  vapours  of 
the  same  nature  as  their  own.  There  are  some  of  them  which  are 
known  as  drying  oils^  that  become  thicker  in  the  air  ;  but  this  is  in 
consequence  of  their  having  absorbed  oxygen,  and  not  in  con- 
sequence of  evaporation. 

Some  substances  give  vapours  even  in  the  solid  state.  Ice  gives 
an  instance  of  this,  as  is  seen  in  dry  cold  winters,  where  the  snow 
and  ice  quite  disappear  from  the  ground,  without  there  having  been 
any  fusion.  Camphor  and  odoriferous  bodies,  in  general  present 
the  same  phenomenon. 

238.  Elastic  force  of  vapours. — Vapours  formed  on  the  surface 
of  a  liquid  are  disengaged  in  virtue  of  their  elasticity  ;  but  this 
force  is  generally  far  lower  than  the  pressure  of  the  atmosphere, 
and  hence  liquids  exposed  to  the  air  only  evaporate  slowly. 

The  following  experiment  renders  evident  the  elastic  force  of 
vapours.  A  bent  glass  tube  has  the  shorter  limb  closed  (fig.  194)  ; 
this  branch  and  part  of  the  longer  are  filled  with  mercury.  A 
drop  of  ether  is  then  passed  into  the  closed  leg,  which  in  virtue 


232  On  Heat.  [238- 

of  its  lower  density  rises  to  the  top  of  the  tube  at  B.     The  tube 
thus  arranged  is  immersed  in  a  water  bath  at  a  temperature  of 

about  45°.  The  mercury  then  sinks 
slowly  in  the  short  branch,  and 
the  space  AB  is  filled  with  a  gas 
which  has  all  the  appearance  of 
air.  This  gas  or  aeriform  fluid  is 
nothing  but  the  vapour  of  ether, 
whose  elastic  force  counterbalances 
not  only  the  pressure  of  the  column 
of  mercury  CA,  but  also  the  atmo- 
spheric pressure  exerted  at  C. 

If  the  water  in  the  vessel  be 
cooled,  or  if  the  tube  be  withdrawn, 
the  mercury  gradually  rises  in  the 
short  leg,  and  the  drop  of  liquid 
which  seemed  almost  to  have  dis- 
appeared is  re-formed.  If,  on  the 
contrary,  the  water  in  which  the 
tube  is  immersed  be  still  more 
heated,  the  drop  diminishes  and 
the  mercury  descends  further  in 
the  short  leg  ;  thus  showing  that 
fresh  vapours  are  formed,  and  that 
the  elastic  force  increases.  This 
increase  of  tension  with  the  tempe- 
rature continues  as  long  as  any 
liquid  remains  to  be  vaporised. 
The  crackling  of  wood  in  fires  is  due  to  the  increased  tension 
of  the  vapours  and  gases  formed  in  the  pores  of  the  wood  during 
combustion.  In  roasting  chesnuts  it  is  usual  to  slit  the  outer 
skin  ;  the  object  of  this  is  to  allow  the  vapour  formed  to  escape, 
for  otherwise  it  would  be  liable  to  acquire  such  a  tension  as  to 
burst  the  chesnut  and  scatter  the  particles  far  and  wide. 

239.  Formation  of  vapours  in  a  vacuum. — In  the  previous 
experiment  the  liquid  changed  very  slowly  into  the  state  of  vapour  ; 
the  same  is  the  case  when  a  liquid  is  freely  exposed  to  the  air. 
In  both  cases  the  atmosphere  is  an  obstacle  to  the  vaporisa- 
tion. In  a  vacuum  there  is  no  resistance,  and  the  formation  of 
vapours  is  instantaneous,  as  is  seen  in  the  following  experiment. 
Four  barometer  tubes,  filled  with  mercury,  are  immersed  side  by 


-240]         Formation  of  Vapours  in  a  Vacuum.  233 


side  in  the  same  trough  (fig.  195).  One  of  them,  A,  serves  as  a 
barometer,  that  is,  only  contains  dry  mercury,  and  a  few  drops  ot 
water,  alcohol,  and  ether  are  respectively  introduced  into  the  tubes, 
B,  C,  D.  When  the  liquids  reach  the  vacuum  a  depression  of  the 
mercury  is  at  once  pro- 
duced. But  this  depres- 
sion cannot  be  produced 
by  the  weight  of  the  liquid, 
for  it  is  but  an  infinitely 
small  fraction  of  the 
weight  of  the  displaced 
mercury.  Hence  in  the 
case  of  each  liquid,  some 
vapour  must  have  been 
formed  whose  elastic  force 
has  depressed  the  mer- 
curial column,  and  as  the 
depression  is  greater  in 
the  tube  D  than  in  the 
tube  C,  and  greater  in 
this  than  in  the  tube  B,  it 
is  concluded  that,  for  the 
same  temperature,  the 
elastic  force  of  ether  is 
greater  than  that  of  alco- 
hol vapour,  and  that 
this  in  turn  has  a  greater 
elastic  force  than  that  of 
water.  If  the  depression 


Fig.  195- 


be  measured  by  means  of  a  graduated  scale,  it  will  be  found  that 
at  a  temperature  of  20°  the  elastic  force  of  ether  is  twenty-five 
times  as  great  as  that  of  water,  and  that  of  alcohol  almost  four 
times  as  great.  From  these  experiments  we  obtain  trie  two  follow- 
ing laws  for  the  formation  of  vapours  : 

I.  In  a  vacuum  all  volatile  liquids  are  instantaneously  converted 
into  vapour. 

II.  At  the  same  temperature  the  vapours  of  different  liquids  have 
different  elastic  forces. 

240.  Iiimit  to  the  formation  and  to  the  tension  of  vapours. 
Saturated  space. — The  quantity  of  vapour  which  can  be  formed 
in  a  given  space,  whether  at  the  ordinary  or  at  higher  temperatures, 


234  On  Heat.  [240- 

is  always  limited.  For  instance,  in  the  above  experiment,  the  de- 
pression of  mercury  in  each  tube,  B,  C,  D,  is  not  stopped  for  want 
of  liquid  which  might  form  fresh  vapours,  for  care  is  taken  always 
to  add  so  much  that  a  slight  excess  remains  unvaporised.  Thus, 
in  the  tube  D,  enough  ether  is  left  ;  yet  we  might  wait  weeks  and 
years,  and  if  the  temperature  did  not  increase,  we  should  always 
see  a  portion  of  liquid  in  the  tube,  and  the  level  of  the  mercury 
remain  stationary.  This  shows  that  no  new  vapours  can  be  formed 
in  the  tube,  and  at  the  same  time  that  the  elastic  force  of  the 
vapour  which  is  there  cannot  increase,  which  is  expressed  by  saying 
that  it  has  attained  its  maximum  tension. 

When  a  given  space  has  acquired  all  the  vapour  which  it  can 
contain,  it  is  said  to  be  saturated.  For  instance,  if  in  a  bottle  full 
of  dry  air  a  little  water  be  placed,  and  the  vessel  be  hermetically 
closed,  part  of  the  water  will  evaporate  slowly,  until  the  elastic 
force  of  the  vapour  formed  holds  in  equilibrium  the  expansive  force 
of  that  which  still  tends  to  form  ;  the  formation  of  vapour  then 
ceases,  and  the  space  is  saturated. 

241.  The  quantity  of  vapour  which  saturates  a  given  space 
is  the  same  whether  this  is  vacuous  or  contains  air. — For 
the  same  temperature  the  quantity  of  vapour  necessary  to  saturate 
a  given  space  is  the  same,  whether  the  space  is  quite  vacuous,  or 
contains  air  or  any  other  gas.  In  the  above  bottle,  whether  it  be 
full  of  air,  or  has  been  exhausted,  the  total  quantity  which  evapo- 
rates is  exactly  the  same  ;  the  difference  being  that,  in  the  first 
case,  the  evaporation  only  takes  place  slowly,  while  in  the  second 
case  it  is  instantaneous.  Yet,  for  the  same  space,  whether  it  be 
vacuous  or  full  of  air,  the  quantity  of  vapour  formed  which  corre- 
sponds to  the  state  of  saturation,  varies  with  the  temperature. 
The  higher  the  temperature,  the  greater  is  the  quantity  of  vapour 
contained  in  a  given  space,  the  denser  it  is  therefore  ;  on  the  other 
hand,  the  lower  the  temperature,  the  less  is  the  quantity  required 
to  saturate  a  given  space. 

The  quantity  of  vapour  present  in  air  is  very  variable  ;  but,  spite 
of  the  abundant  vaporisation  produced  on  the  surface  of  seas, 
lakes,  and  rivers,  the  air  in  the  lower  regions  of  the  atmosphere  is 
never  quite  saturated,  even  when  it  rains.  This  arises  from  the  fact, 
that  aqueous  vapour  being  less  dense  than  air,  in  proportion  as 
it  is  formed,  rises  into  the  higher  regions  of  the  atmosphere,  where, 
condensed  by  cooling,  it  falls  as  rain. 


-242]  Evaporation.  235 

242.  Evaporation.  Causes  which  accelerate  it. — We  have 
hitherto  designated,  under  the  general  term  of  vaporisation,  all 
production  of  vapour  under  whatever  circumstances  it  takes  place, 
whether  slow  or  rapid,  whether  in  air  or  in  a  vacuum  ;  while  the 
term  evaporation  is  especially  assigned  to  the  slow  formation  of  a 
vapour  on  the  surface  of  a  volatile  liquid  when  it  is  exposed  in  the 
open  air.  It  is  in  consequence  of  evaporation  that  the  level 
gradually  diminishes  in  a  pond  full  of  water,  and  ultimately  dries 
up  if  it  is  not  fed  by  a  spring.  Owing  to  the  same  cause  the  earth 
moistened  by  rain  dries  up  and  ultimately  hardens  ;  that  moist 
linen  exposed  in  the  air  soon  dries  up.  Several  causes  influence 
the  rapidity  of  the  evaporation  of  a  liquid  :  the  temperature  ;  the 
quantity  of  the  same  vapour  in  the  surrounding  atmosphere  ;  the 
renewal  of  this  atmosphere ;  the  extent  of  the  surface  of  evapora- 
tion. 

Influence  of  temperature.  Heat  being  the  agent  of  all  evapora- 
tion, the  higher  the  temperature  the  more  abundant  is  the  forma- 
tion of  vapour.  This  property  is  utilised  in  the  arts  to  hasten  and 
complete  the  drying  of  a  large  number  of  products  which  are 
exposed  in  stoves  ;  that  is  to  say,  in  chambers,  the  temperature 
of  which  is  kept  at  30,  40,  50,  and  even  60  degrees,  and  the  air 
of  which  is  continually  renewed  to  allow  the  vapour  formed  to 
escape. 

Influence  of  pressure.  We  have  already  seen  that  the  pressure 
of  the  atmosphere  is  an  obstacle  to  the  disengagement  of  vapours, 
and  it  will  thus  be  understood  that  when  this  pressure  is  diminished 
they  ought  to  be  formed  more  abundantly.  This,  in  point  of  fact, 
is  what  takes  place  whenever  liquids  are  removed  from  the  pressure 
of  the  atmosphere.  In  sugar  refineries,  in  order  to  concentrate 
the  syrups  (that  is,  to  reduce  the  volume  by  removing  part  of  the 
water  they  contain),  they  are  placed  in  large  spherical  vessels  ;  and 
then,  by  the  aid  of  large  air-pumps  of  special  construction,  and 
worked  by  steam  engines,  the  air  in  the  boilers  is  rarefied,  which 
considerably  accelerates  the  evaporation  of  water,  and  quickly 
brings  the  syrups  to  the  wished-for  degree  of  concentration. 

Influence  of  the  renewal  of  air.  In  order  to  understand  the  in- 
fluence of  the  third  cause,  it  is  to  be  observed  that  no  evaporation 
could  take  place  in  a  space  already  saturated  with  vapour  of  the 
same  liquid,  and  that  it  would  reach  its  maximum  in  air  com- 
pletely freed  from  this  vapour.  It  therefore  follows  that,  between 


On  Heat. 


[242- 


these  two  extremes,  the  rapidity  of  evaporation  varies  according  as 
the  surrounding  atmosphere  is  already  more  or  less  charged  with 
the  same  vapour. 

The  effect  of  the  renewal  of  this  atmosphere  is  similarly  ex- 
plained ;  for  if  the  air  or  gas,  which  surrounds  the  liquid,  is  not 
renewed,  it  soon  becomes  saturated,  and  evaporation  ceases. 

Thus  it  is  that  the  wind,  removing  the  layers  of  air  which  are  in 
contact  with  the  earth,  soon  dries  up  the  roads  and  streets.  Hence, 
too,  it  is  that  linen  hung  out  to  dry,  does  so  far  more  rapidly  on  a 
windy  than  on  a  dry  day. 

The  greater  the  extent  of  surface  which  a  liquid  presents  to  the 
air,  the  more  numerous  are  the  points  from  which  vapour  is  dis- 
engaged. Hence  the  evaporation  of  a  liquid  should  be  effected  in 

vessels  which  are  wide 
and  shallow.  This  is 
what  is  done  in  the 
process  of  extracting 
salt  from  sea  water  in 
salt  gardens.  The  sea 
water  is  admitted  into 
broad  and  shallow 
pits  excavated  in  the 
ground.  Under  the 
influence  of  the  sun's 
heat  the  water  evapo- 
rates slowly,  and  when 
its  concentration  has 
reached  the  point  at 
which  the  liquid  is 
saturated,  the  salt  then 
begins  to  form  on  the 
surface  and  is  raked  off. 
243.  Ebullition. — 
Ebullition,  or  boiling, 
is  the  rapid  production 
of  elastic  bubbles  of 
vapour  in  the  mass  of 
a  liquid  itself. 

When  a  liquid,  water  for  example,  is  heated  at  the  lower  part  of 
a  vessel,  the  first  bubbles  are  due  to  the  disengagement  of  air 
which  had  previously  been  absorbed.  Small  bubbles  of  vapour 


Fig.  196. 


-245]       Causes  which  Influence  the  Boiling  Point.        237 

then  begin  to  rise  from  the  heated  parts  of  the  sides,  but  as  they 
pass  through  the  upper  layers,  the  temperature  of  which  is  lower, 
they  condense  before  reaching  the  surface.  The  formation  and 
successive  condensation  of  these  first  bubbles  occasion  the  singing 
noticed  in  liquids  before  they  begin  to  boil.  Lastly,  large  bubbles 
rise  and  burst  on  the  surface,  and  this  constitutes  the  phenomenon 
of  ebullition  (fig.  196). 

244.  liaws   of  ebullition. — The  laws  of  ebullition  have  been 
determined  experimentally,  and  are  as  follows  : 

I.  The  temperature  of  ebullition,  or  the  boiling  point,  increases 
ivith  the  pressure. 

II.  For  a  given  pressure  ebullition  commences  at  a  certain  tem- 
perature, which  varies  in  different  liquids,  but  which,  for  equal 
pressures,  is  always  the  same  in  the  same  liquid. 

III.  Whatever  be  the  intensity  of  the  source  of  heat,  as  soon  as 
ebullition  commences,  the  temperature  of  the  liquid  remains  sta- 
tionary. % 

Thus,  the  boiling  point  of  water  under  the  ordinary  atmospheric 
pressure  being  100°,  it  could  not  be  heated  beyond  that  point, 
whatever  the  intensity  of  the  source  of  heat ;  hence  all  the  heat 
which  passes  from  the  source  into  the  liquid  is  absorbed  by  the 
vapour  disengaged.  But,  as  this  vapour  is  itself  at  100°,  we  must 
conclude  that  this  heat  is  not  absorbed  to  raise  the  temperature  of 
the  vapour,  but  simply  to  produce  it ;  that  is,  to  change  the  sub- 
stance from  the  liquid  into  the  gaseous  state,  a  phenomenon  analo- 
gous to  that  which  fusion  presents  (232).  This  disappearance  of 
heat  during  ebullition  will  be  subsequently  investigated  under  the 
name  of  latent  heat  of  vaporisation  (250). 

Boiling  points  under  the  pressure  of  an  atmosphere. 

Sulphurous  acid     .     .     .  — 10°  Turpentine 160° 

Ether 37  Strong  sulphuric  acid  .     .     325 

Bisulphide  of  carbon  .     .      48  Mercury 350 

Bromine 63  Sulphur 447 

Alcohol 78  Cadmium.     .     .     .  'V  '."860 

Distilled  water   ....     100  Zinc 1040 

245.  Causes  which  influence  the  boiling-  point. — The  boiling 
point  of  a  liquid  is  affected  by  the  substances  in  solution,  by  the 
degree  of  pressure  to  which  it  is  subjected,  and  by  the  nature  of 
the  vessels  in  which  the  boiling  takes  place. 


238 


On  Heat. 


[245- 


The  ebullition  of  a  liquid  is  the  more  retarded,  the  greater  the 
quantity  of  any  substance  it  may  contain  in  solution,  provided  that 
the  substance  be  not  volatile,  or,  at  all  events,  be  less  volatile  than 
the  liquid  itself.  Water  which  boils  at  100°  when  pure,  boils  at 
109°  when  it  is  saturated  with  common  salt ;  that  is,  when  it  has 
taken  up  as  much  of  this  salt  as  it  can  dissolve.  Fatty  matters 
combined  with  water  also  raise  its  boiling  point ;  hence  it  is  that 
fat  soup  burns  more  severely  than  water. 


Fig.  197. 

Pressure.  The  degree  of  pressure  to  which  a  liquid  is  subjected 
has  a  most  important  influence  on  its  boiling  point  The  greater 
the  pressure  the  greater  must  be  the  tension,  in  order  that  the 
vapour  may  be  disengaged,  and  therefore  the  higher  the  temperature 
On  the  contrary,  the  less  the  pressure,  the  lower  the  temperature 
at  which  ebullition  takes  place.  If  the  pressure  of  the  atmosphere 
be  removed,  water  may  be  made  to  boil,  even  at  the  ordinary  tern- 


—245]     Influence  of  P  res  stir e  on  the  Boiling  Point.       239 

perature.  The  experiment  may  be  arranged  in  the  manner  repre- 
sented in  fig.  197.  A  glass  cup  containing  water  is  placed  under 
the  bell-jar  of  an  air-pump,  or,  in  order  that  the  experiment  may  be 
seen  by  a  number  of  spectators,  the  bell  is  placed  on  a  movable 
plate  connected  with  the  pump  by  a  tube.  When  a  vacuum  is  pro- 
duced, or  when  the  air  is  very  rarefied,  the  water  is  seen  to  boil, 
evidently  indicating  a  considerable  disengagement  of  vapour.  Yet 
the  temperature  of  the  liquid  is  not  raised  ;  the  boiling  is,  on  the 
contrary,  a  source  of  cold,  owing  to  the  heat,  which  becomes  latent 
in  the  formation  of  vapours. 

The  influence  of  pressure  on  ebullition  may  further  be  illustrated 
by  means  of  an  experiment  of  Franklin's.  The  apparatus  consists 
of  a  bulb  and  a  tube,  joined  by  a  tube  of  smaller  dimensions  (fig. 
198).  The  tube  is  drawn  out,  and  the  apparatus  filled  with  water, 
which  is  then  in  great  part  boiled  away  by  means  of  a  spirit-lamp. 


Fig.  198. 

When  it  has  been  boiled  sufficiently  long  to  expel  all  the  air,  the 
tube  is  sealed.  There  is  then  a  vacuum  in  the  apparatus,  or  rather, 
there  is  only  a  pressure  due  to  the  tension  of  aqueous  vapour, 
which  at  ordinary  temperatures  is  very  small.  Consequently,  if  the 
bulb  be  placed  in  the  hand,  as  shown  in  the  figure,  the  heat  is 
sufficient  to  produce  a  pressure,  which  drives  the  water  into  the 
tube  and  causes  a  brisk  ebullition. 

A  paradoxical  but  very  simple  experiment  also  well  illustrates 
the  dependence  of  the  boiling  point  on  the  pressure.  In  a  glass 
flask  water  is  boiled  for  some  time,  and  when  all  air  has  been  ex- 
pelled by  the  steam,  the  flask  is  closed  by  a  cork  and  inverted,  as 
shown  in  fig.  199.  If  the  bottom  is  then  cooled  by  a  stream  of 


240 


On  Heat. 


[245- 


cold  water  from  a*  sponge,  the  water  begins  to  boil  again.  This 
arises  from  the  condensation  of  the  steam  above  the  surface  of  the 
water,  by  which  a  partial  vacuum  is  produced. 

As  the  pressure  of  air  diminishes  in  proportion  as  we  rise  in  the 
atmosphere,  it  will  be  seen  from  what  has  been  said,  that  on  high 

mountains  water  must  boil  at 
lower  temperatures  than  on 
the  sea  level.  This,  in  fact, 
is  the  case  ;  on  Mont  Blanc, 
at  a  height  of  15,800  feet, 
water  boils  at  84°  ;  at  Quito, 
at  a  height  of  11,000  feet, 
at  90°  ;  and  at  Madrid,  the 
height  of  which  is  3,000 
feet,  it  boils  at  97°.  This 
diminution  in  the  tempera- 
ture of  ebullition  at  great 
heights  is  a  material  obstacle 
to  the  preparation  of  food, 
for,  at  the  temperature  of  90°, 
the  extraction  of  the  nourish- 
ment and  of  the  flavour  is  far 
more  imperfect  than  under 
the  usual  conditions. 

In    deep   mines,   on   the 
contrary,    such   as    those    of 
Fig-  X99-  Cornwall     and     Lancashire, 

the  reverse  is  the  case  :  the  pressure  increases  with  the  depth,  and 
the  boiling  point  is  higher  than  at  100°. 

Influence  of  the  nature  of  the  vessel  on  the  boiling  point.  Gay- 
Lussac  observed  that  water  in  a  glass  vessel  required  a  higher 
temperature  for  ebullition  than  in  a  metal  one.  Taking  the  tem- 
perature of  boiling  water  in  a  copper  vessel  at  100°,  its  boiling 
point  in  a  glass  vessel  was  found  to  be  101°  ;  and  if  the  glass 
vessel  had  been  previously  cleaned  by  means  of  sulphuric  acid  and 
of  potass,  the  temperature  would  rise  to  105°  or  even  to  106°  before 
ebullition  commenced.  Whatever  be  the  boiling  point  of  water, 
the  temperature  of  its  vapour  is  uninfluenced  by  the  substance  of 
the  vessels. 

246.  Papin's  digester. — What  has  hitherto  been  said  in  re- 
ference to  the  formation  of  vapour  has  applied  to  the  case  of  liquids 
heated  in  open  vessels.  Only  under  these  conditions  can  ebulli- 


-246] 


Papiiis  Digester. 


241 


tion  take  place  ;  for,  in  a  closed  vessel,  since  the  vapours  cannot 
escape  into  the  atmosphere,  their  elastic  force  and  their  density 
continually  increase,  but  that  peculiarly  rapid  disengagement  which 
constitutes  boiling  is  impossible.  There  is,  moreover,  this  differ- 
ence between  heating  in  an  open  and  in  a  closed  vessel  ;  that,  in  the 
former  case,  the  temperature  can  never  exceed  that  of  ebullition, 
while  in  a  closed  vessel  it  may  be  raised  so  to  speak,  to  an  in- 
definite extent.  Thus  we  have  seen  (246)  that,  in  an  open 
vessel,  water  cannot  be  heated  beyond  100°  C.,  all  the  heat  im- 
parted to  it  being  absorbed  by  the  vapours  disengaged.  But  as 
this  disengagement  of  vapour 
cannot  take  place  in  a  closed 
vessel,  water  and  the  vapour 
may  be  raised  to  a  far  higher 
temperature  than  100°.  Yet 
this  is  not  unattended  with 
danger,  from  the  very  high 
tension  which  the  vapour 
then  assumes. 

Figure  200  represents  the 
apparatus  used  in  physical 
lectures  for  the  purpose  of 
heating  water  in  a  closed 
vessel  beyond  100  degrees. 
It  is  known  as  Papin's  Di- 
gester. It  consists  of  a 
cylindrical  bronze  vessel, 
M  (fig.  200),  provided  with 
a  cover,  which  is  firmly 
fastened  down  by  a  screw. 
In  order  to  close  the  vessel 
hermetically,  sheet  lead  is  Fig-  2°°- 

placed  between  the  edges  of 

the  cover  and  the  vessel.  At  the  bottom  of  a  cylindrical  cavity, 
which  traverses  a  cylinder  and  tubulure,  the  cover  is  perforated  by  a 
small  orifice  in  which  there  is  a  rod,  u.  This  rod  presses  against  a 
lever,  ab,  movable  at  a,  and  the  pressure  may  be  regulated  by 
means  of  a  weight,  /,  movable  on  this  lever.  The  lever  is  so 
weighted,  that  when  the  tension  in  the  interior  is  equal  to  six 
atmospheres,  for  example,  the  valve  rises  and  the  vapour  escapes; 
The  destruction  of  the  apparatus  is  thus  avoided,  and  the  mecha- 

R 


242 


On  Heat. 


[246- 


nism,  which  will  be  described  in  speaking  of  the  steam  engine 
(269)  has  hence  received  the  name  (A  safety  valve.  The  digester 
is  filled  about  two-thirds  with  water,  and  is  heated  on  a  furnace. 
The  water  may  thus  be  raised  to  a  temperature  far  above  100°,  and 
the  tension  of  the  vapour  increased  to  several  atmospheres,  accord- 
ing to  the  weight  on  the  lever. 

The  apparatus  has  received  the  name  digester,  from  a  Latin  word 
signifying  to  dissolve,  for  the  high  temperature  which  water  can 
acquire  greatly  increases  its  solvent  power.  Thus  it  is  used  to 

extract  from  bones  the  substance 
known  as  glue,  which  could  not 
be  accomplished  at  100°. 

From  the  enormous  elastic 
force  which  vapour  may  acquire 
in  a  closed  vessel,  it  will  be 
understood  how  important  it  is 
not  to  close  tightly  the  ves- 
sel in  which  water  is  contained 
for  domestic  purposes.  Thus  a 
hot  water-bottle  for  heating  the 
feet  of  invalids  should  be  un- 
corked before  being  placed  near 
the  fire  ;  for  it  might  burst,  or 
at  any  rate  the  cork  might  be 
driven  out,  and  a  more  or  less 
serious  accident  be  caused.  In 
like  manner,  when  a  locomotive 
stops,  the  steam  must  be  allowed 
to  escape  ;  for  otherwise,  as  it  is 
continually  being  formed  in  the 
boiler  without  any  being  con- 
sumed in  working  the  engine,  it 
would  ultimately  acquire  such 
an  elastic  force  that  an  explosion 
would  ensue. 

247.  Measurement  of  the 
elastic  force  of  aqueous  va- 
pour.— The  important  applica- 


Fig.  201, 


tions  which  have  been  made  of  the  elastic  force  of  aqueous  vapour, 
have  led  philosophers  to  measure  with  care  the  intensity  of  this  force 
at  various  temperatures. 

Dalton  first  measured  the  elastic  force  of  aqueous  vapour  for 


-247]  Elastic  Force  of  Aqueous  Vapour. 


243 


temperatures  between  o°  and  100°,  by  means  of  the  apparatus  re- 
presented in  fig.  201.  Two  barometer  tubes,  A  and  B,  are  filled 
with  mercury,  and  inverted  in  an  iron  bath  full  of  mercury,  and 
placed  on  a  furnace.  The  tube,  A,  is  an  ordinary  barometer  tube, 
freed  from  air  and  moisture  ;  but  into  the  tube,  B,  is  introduced  a 
small  quantity  of  water.  The  tubes  are  supported  in  a  cylindrical 
vessel  full  of  water,  the  temperature  of  which  is  indicated  by  the 
thermometer  /.  The  bath  being  gradually  heated,  the  water  in  the 
cylinder  becomes  heated  too  :  the  water  which  is  in  the  tube  B 
vaporises,  and  in  proportion  as  the  elastic  force  of  its  vapour  in- 
creases, the  mercury  sinks.  The  depressions  of  the  mercury  cor- 
responding to  each  degree  of  the  thermometer,  are  indicated  on  the 
scale.  Thus,  if,  when  the  thermometer  is  at  70°,  the  mercury  is  233 
millimetres  lower  in  the  tubs  B  than  in  the  tube  A,  this  shows  that  at 
70°  the  tension  of  aqueous  vapour  is  233  millimetres  ;  which  amounts 
to  saying  that  it  exercises  on  the  sides  of  the  vessel  which  contains  it 
a  pressure  equal  to  the  weight  of  a  column  of  mercury  233  millimetres 
in  height. 

By  noting  in  the  above  manner  the  depression  in  the  baro- 
meter, B,  as  compared  with  A,  Dalton  determined  the  elastic  force 
of  aqueous  vapour  from  o°  to  100°.  He  found  it  to  be  760  milli- 
metres, or  29-92  inches  ;  that  is  to  say,  an  atmosphere  (118). 

Dulong  and  Arago  determined  the  elastic  force  of  aqueous 
vapour  above  100°  up  to  pressures  of  24  atmospheres.  More 
recently  Regnault  measured  the  elastic  force  of  aqueous  vapour 
both  above  and  below  100° ;  and  from  the  researches  of  this 
experimenter  the  following  table  has  been  taken,  in  which  the  elas- 
tic forces  at  various  temperatures  are  respectively  measured  by  the 
height  in  millimetres  of  the  column  of  mercury  which  they  can 
balance. 

Elastic  force  of  aqueous  vapour. 


Temperatures 

Tensions  in 
millimetres 

Temperatures 

Tensions  in 
millimetres 

0 

4'6o 

60 

14879 

5 

6'53 

70 

233-09 

10 

9*i7 

80 

354^4 

15 

1270 

90 

525^5 

20 

1  7  '39 

JOO 

760-00 

30 

3i'5S 

101 

787-63 

40 

54'9J 

1  20 

I520-OO 

50 

91-98 

160 

458O-OO 

R  2 


244  On  Heat.  [247- 

This  table  shows  that  the  elastic  force  of  aqueous  vapour  in- 
creases far  more  rapidly  than  the  temperature.  Thus  at  50°  the 
tension  is  only  91-9  millimetres;  while  at  100  degrees,  that  is  to 
say,  double  the  temperature,  the  tension  is  eight  times  as  great. 

248.  Latent  beat  of  vapour. — In  speaking   of  ebullition    we 
have  seen  that,  from  the  moment  a  liquid  begins  to  boil,  its  tempera- 
ture ceases  to  rise  whatever  be  the  intensity  of  the  source  of  heat. 
It  follows  that  a  considerable  quantity  of  heat  becomes  absorbed  in 
ebullition,  the  only  effect  of  which  is  to  transform  the  body  from 
the    liquid  to  the    gaseous   condition.     And   conversely,   when   a 
saturated  vapour  passes  into  the   state  of  liquid,  it  gives  out  an 
amount  of  heat. 

These  phenomena  were  first  observed  by  Black,  and  he  des- 
cribed them  by  saying  that,  during  vaporisation,  a  quantity  of  sen- 
sible heat  became  latent,  and  that  the  latent  heat  again  became  free 
during  condensation.  The  quantity  of  heat  which  a  liquid  must 
absorb  in  passing  from  the  liquid  to  the  gaseous  state,  and  which 
it  gives  out  in  passing  from  the  state  of  vapour  to  that  of  liquid,  is 
spoken  of  as  the  latent  heat  of  evaporation. 

The  analogy  of  these  phenomena  to  those  of  fusion  will  be  at 
once  seen.  The  modes  of  determining  them  need  not  be  described  ; 
but  the  following  results  which  have  been  obtained  for  the  latent 
heats  of  evaporation  of  a  few  liquids  may  be  here  given  : — 

Water  .  .  . '  /  .  540  Bisulphide  of  carbon  .  .  87 
Alcohol  ....  208  Turpentine  ",'"'  .  .  74 
Ether  .  .  .  .90  Bromine  .  .  .  .46 

The  meaning  of  these  numbers  is,  in  the  case  of  water,  for  in- 
stance, that  it  requires  as  much  heat  to  convert  a  pound  of  water 
from  the  state  of  liquid  at  the  boiling  point  to  that  of  vapour  at  the 
same  temperature,  as  would  raise  a  pound  of  water  through  540 
degrees,  or  540  pounds  of  water  through  one  degree  ;  or  that  the 
conversion  of  one  pound  of  vapour  of  alcohol  at  78°  into  liquid 
alcohol  of  the  same  temperature  would  heat  208  pounds  of  water 
through  one  degree. 

249.  Cold  due  to  evaporation. — Whatever  be  the  temperature 
at  which  a  vapour  is  produced,  an  absorption  of  heat  always  takes 
place.     If,  therefore,  a  liquid  evaporates,  and  does  not  receive  from 
without  a  quantity  of  heat  equal  to  that  which  is  expended  in  pro- 
ducing the  vapour,  its  temperature  sinks,  and  the  cooling  is  greater 
in  proportion  as  the  evaporation  is  more  rapid. 

'"• 


-250] 


Water  Frozen  in  a  Vacuum. 


245 


This  may  become  a  source  of  very  great  cooling.  Thus  if  a 
few  drops  of  ether  be  placed  on  the  hand,  and  this  be  agitated  to 
accelerate  the  evaporation,  great  cold  is  experienced.  With 
liquids  which  are  less  volatile  than  ether,  like  alcohol  and  water, 
the  same  phenomenon  is  produced,  but  the  cooling  is  less  marked. 
On  coming  out  of  a  bath,  and  more  especially  in  the  open  air 
and  with  some  wind,  a  very  sharp  cold  is  experienced,  due  to  the 
vapour  formed  on  the  surface  of  the  body.  Moist  linen  is  cold  and 
injurious,  because  it  withdraws  from  the  body  the  heat  which  the 
moisture  requires  for  evaporation. 

The  cooling  effect  produced  by  a  wind  or  draught  does  not 
necessarily  arise  from  the  wind  being  cooler,  for  it  may,  as  shown 
by  the  thermometer,  be  actually  warmer ;  but  arises  from  the 
rapid  evaporation  it  causes  from  the  surface  of  the  skin.  We  have 
the  feeling  of  oppression,  even  at  moderate  temperatures,  when  we 
are  in  an  atmosphere  saturated  by  moisture  in  which  no  evaporation 
takes  place. 

The  cooling  produced  by  the  use  of  fans  is  due  to  the  increased 
evaporation  they  produce.  The  freshness  occasioned  by  watering 
the  streets  is  also  an  effect  of  evaporation. 

The  cold  produced  by  evaporation  is  used  in  hot  climates  to  cool 
water  by  means  of  alcarrazas.  These  are  porous  earthen  vessels, 
through  which  water  percolates,  so  that 
on  the  outside  there  is  a  continual  evapo- 
ration, which  is  accelerated  when  the 
vessels  are  placed  in  a  current  of  air.  For 
the  same  reason  wine  is  cooled  by  wrap- 
ping the  bottles  in  wet  cloths  and  placing 
them  in  a  draught. 

250.  Water  and  mercury  frozen  in 
a  vacuum. — From  the  great  quantity  of 
heat  which  disappears  when  a  liquid 
is  converted  into  vapour  it  will  be  seen 
that  by  accelerating  the  evaporation  we 
have  a  means  of  producing  intense  cold. 
We  have  seen  that  liquids  vaporise  more 
rapidly  the  lower  the  pressure.  Hence,  if 
a  vessel  containing  water  be  placed  in  a  Fig.  202. 

space  from  which  the  air  is  exhausted,  it  should  cool  very  rapidly. 

Leslie  succeeded  in  freezing  water  by  means  of  rapid  evapora- 
tion.    Under  the  receiver  of  the  air-pump  is  placed  a  vessel  con- 


246  On  Heat.  [250- 

taining  strong  sulphuric  acid,  a  substance  which  has  a  great  affi- 
nity for  water,  and  above  it  a  thin,  shallow,  porous  capsule  (fig. 
202)  containing  a  small  quantity  of  water.  By  exhausting  the 
receiver  the  water  begins  to  boil,  and  since  the  vapours  are  absorbed 
by  the  sulphuric  acid  as  fast  as  they  are  formed,  a  rapid  evaporisa- 
tion  is  produced,  which  quickly  effects  the  freezing  of  the  water. 

By  using  liquids  more  volatile  than  water,  more  particularly 
liquid  sulphurous  acid,  which  boils  at  — 10°,  a  degree  of  cold  is 
obtained  sufficiently  intense  to  freeze  mercury.  The  experiment 
may  be  made  by  covering  the  bulb  of  a  thermometer  with  cotton 
wool,  and  after  having  moistened  it  with  liquid  sulphurous  acid, 
placing  it  under  the  receiver  of  the  air  pump.  When  a  vacuum  is 
produced  the  mercury  is  quickly  frozen. 

Thilorier,  by  directing  a  jet  of  liquid  carbonic  acid  on  the  bulb 
of  an  alcohol  thermometer,  obtained  a  cold  of  —  100°  without  freez- 
ing the  alcohol.  With  a  mixture  of  solid  carbonic  acid,  liquid  pro- 
toxide of  nitrogen  and  ether,  M.  Despretz  obtained  a  sufficient 
degree  of  cold  to  reduce  alcohol  to  the  viscous  state. 

By  means  of  the  evaporation  of  bisulphide  of  carbon  the  forma- 
tion of  ice  may  be  illustrated  without  the  aid  of  an  air-pump.  A 
little  water  is  dropped  on  a  small  piece  of  wood,  and  a  capsule  of 
thin  copper  foil,  containing  bisulphide  of  carbon,  is  placed  on  the 
water.  The  evaporation  of  the  bisulphide  is  accelerated  by  means 
of  a  pair  of  bellows,  and  after  a  few  minutes  the  water  freezes  round 
the  capsule,  so  that  the  latter  adheres  to  the  wood. 


CHAPTER   VIII. 
LIQUEFACTION   OF  VAPOURS  AND  GASES. 

251.  liquefaction  of  vapours. — The  liquefaction  or  condensa- 
tion of  vapours  is  their  passage  from  the  aeriform  to  the  liquid 
state.  Condensation  may  be  due  to  three  causes — cooling,  com- 
pression, or  chemical  affinity. 

When  vapours  are  condensed,  their  latent  heat  becomes  free, 
that  is,  it  affects  the  thermometer.  This  is  readily  seen  when  a 
current  of  steam  at  100°  is  passed  into  a  vessel  of  water  at  the  ordi- 
nary temperature.  The  liquid  becomes  rapidly  heated,  and  soon 
reaches  100°.  The  quantity  of  heat  given  up  in  liquefaction  is  equal 
to  the  quantity  absorbed  in  producing  the  vapour. 


-251]  Liquefaction  of  Gases.  247 

Liquefaction  by  chemical  affinity. — The  affinity  of  certain  sub- 
stances for  water  is  so  great  as  to  condense  the  vapours  in  the 
atmosphere,  even  when  they  are  far  from  their  point  of  saturation. 
Thus,  when  highly  hygroscopic  substances,  such  as  quicklime, 
potass,  sulphuric  acid,  are  exposed  in  the  air,  they  always  absorb 
aqueous  vapour.  Certain  varieties  of  common  salt  exposed  to  the 
air  absorb  and  condense  so  much  aqueous  vapour  as  to  become 
liquid.  Many  other  salts  have  the  same  property,  and  are  hence 
called  deliquescent  salts. 

Liquefaction  by  pressure. — Let  us  suppose  a  vessel  containing 
aqueous  vapour,  a  cylinder  for  instance,  and  in  this  cylinder  a 
piston  which  can  be  depressed  at  will,  like  that  represented  in  fig. 
4  page  9.  As  the  vapour  is  not  at  first  in  a  state  of  saturation, 
when  the  piston  is  depressed,  it  behaves  like  a  true  gas,  the  pressure 
increasing  its  elastic  force  and  density  without  liquefying  it.  But 
the  more  the  piston  is  depressed  the  smaller  does  the  volume  of 
the  vapour  become,  and  a  point  is  ultimately  reached  at  which  the 
vapour  present  is  just  sufficient  to  saturate  the  space.  From  this 
point  the  slightest  increase  of  pressure  causes  a  portion  of  vapour 
to  pass  into  the  liquid  state,  and  the  liquefaction  continues  as  long 
as  the  excess  of  pressure  lasts  ;  so  that  if  the  piston  descends  to  the 
bottom  of  the  cylinder  all  the  vapour  is  condensed.  In  this  ex- 
periment it  is  to  be  observed,  that  when  once  saturation  is  attained, 
provided  there  is  no  air  in  the  cylinder,  the  resistance  to  the  de- 
pression of  the  piston  does  not  increase  in  proportion  as  it  descends, 
which  arises  from  the  condensation  of  the  vapour,  and  confirms 
what  was  previously  said  (240),  as  to  the  maximum  tension  of 
vapour  in  a  state  of  saturation. 

Liquefaction  by  cooling. — Cooling,  as  well  as  pressure,  only 
causes  vapours  to  liquefy  when  they  are  in  a  state  of  saturation.  But 
when  once  a  given  space  is  saturated,  the  slightest  lowering  of 
temperature  takes  from  the  vapours  the  heat  which  gives  them  their 
fluidity,  the  attraction  between  the  molecules  preponderates,  they 
agglomerate,  forming  extremely  small  droplets,  which  float  in  the 
air  and  are  deposited  on  the  surrounding  bodies. 

Vapours  are  ordinarily  condensed  by  cooling.  Thus,  the  vapours 
exhnled  from  the  nose  and  mouth  of  animals  first  saturate  the 
colder  air  in  which  they  are  disengaged,  and  they  condense  with  a 
cloud-like  appearance.  It  is  owing  to  the  same  phenomenon  that 
the  vapours  become  visible  which  are  disengaged  from  boiling 
water,  those  which  rise  from  chimneys,  the  fogs  formed  above 


248  On  Heat.  [251- 

rivers,  and  so  forth.  All  these  vapours  are  more  apparent  in  winter 
than  in  summer,  for  then  the  air  is  colder,  and  the  condensation 
more  complete. 

In  cold  weather,  the  windows  in  heated  rooms  are  seen  to  become 
covered  with  dew  on  the  inside.  The  air  of  these  rooms  is  in 
general  far  from  being  saturated  with  vapour,  but  the  layers  of  air 
in  immediate  contact  with  the  windows  become  colder  ;  and  as  the 
quantity  of  vapour  necessary  to  saturate  a  given  space  is  less,  the 
colder  this  space,  a  moment  is  reached  at  which  the  air  in  contact 
with  the  windows  is  saturated,  and  then  the  vapours  they  contain 
are  quickly  deposited.  In  a  time  of  thaw,  when  the  air  is  hotter  on 
the  outside  than  on  the  inside,  the  deposit  is  formed  on  the  outside. 
To  the  same  cause  is  due  the  deposit  of  moisture  formed  on  walls, 
which  is  expressed  by  saying  that  \hzysweat ;  an  unsuitable  expres- 
sion, for  the  moisture  does  not  come  from  the  walls  but  from  the 
atmosphere.  The  walls  are  colder  than  the  air,  and  they  lower  the 
temperature  of  the  layers  in  contact  with  them,  and  condense  the 
vapours.  A  similar  effect  is  produced  when  in  summer  a  bottle  of 
wine  is  brought  from  the  cellar,  or  when  a  glass  is  filled  with  cold 
water ;  a  deposit  of  dew  is  formed  on  the  surface  of  these  vessels. 
The  same  phenomenon  does  not  occur  in  winter,  for  then  the  tem- 
perature of  the  atmosphere  being  the  same  as  that  of  the  bottle,  or 
even  lower,  the  layers  of  air  in  immediate  contact  with  it  are  not 
cooled. 

252.  Heat  disengaged  during:  condensation. — It  has  been  seen 
that  any  liquid  in  vaporising  absorbs  a  quantity  of  heat.     This  heat 
is  not  destroyed,  for,  in  the  converse  change,  it  reappears  in  the 
sensible  state  ;  that  is  to  say,  it  is  capable  of  acting  on  our  sense  of 
feeling  and  on  the  thermometer.      For  instance,  we  know  that  a 
pound  of  water   absorbs  in  vaporising  540  units  of  heat  (248)  ; 
that  is  to  say,  a  quantity  of  heat  necessary  to  raise  540  pounds 
of  water  from  o°  to  i°;  conversely,  a  pound  of  steam  at  100°,  which 
is  liquefied  and  gives  a  pound  of  water  at  100°,  causes  540  units  to 
pass  from  the  latent  to  the  sensible  state,  an  amount  of  heat  which 
is  utilised  in  heating  by  steam. 

253.  Application  to  beating-  by  steam. — The  quantity  of  heat 
which  becomes  free  when  aqueous  vapour  is  condensed  is  utilised  in 
the  arts  for  heating  private  houses,  hot-houses,  and  public  buildings. 
Steam  is  produced  in  boilers  like  those  used  in  steam  engines,  and 
passes  from  thence  into  metal  tubes  concealed  behind  the  wain- 
.scot,  or  into  columns  which  serve  at  the  same  time  as  ornaments 


-254] 


Stills. 


249 


for  rooms.  The  steam  condensing  in  these  tubes  gives  up  a  con- 
siderable quantity  of  heat,  which  they  impart  to  the  surrounding  air. 

254.  Distillation,  stills. — Distillation  is  an  operation  by  which 
volatile  liquid  may  be  separated  from  substances  which  it  holds  in 
solution,  or  by  which  two  liquids  of  different  volatilities  may  be  sepa- 
rated. The  operation  depends  on  the  transformation  of  liquids  into 
vapours  by  the  action  of  heat,  and  on  the  condensation  of  these 
vapours  by  cooling. 

The  apparatus  used  in  distillation  is  called  a  still.  Its  form  may 
vary  greatly,  but  consists  essentially  of  three  parts;  ist,  the  body. 


Fig.  203. 

A  (fig.  203),  a  copper  vessel  containing  the  liquid,  the  lower  part 
of  which  fits  in  the  furnace  ;  2nd,  the  head,  B,  which  fits  on  the 
body,  and  from  which  a  lateral  tube,  C,  leads  to,  3rd,  the  worm, 
S,  a  long  spiral  tin  or  copper  tube,  placed  in  a  cistern  kept  con- 
stantly full  of  cold  water.  The  object  of  the  worm  is  to  condense 
the  vapour,  by  exposing  a  greater  extent  of  cold  surface. 

To  free  ordinary  well  water  from  the  many  impurities  which  it 
contains,  it  is  placed  in  a  still  and  heated.  The  vapours  disen- 
gaged are  condensed  in  the  worm,  and  the  distilled  water  arising 
from  the  condensation  is  collected  in  the  receiver,  D.  The  vapours 
in  condensing  rapidly  heat  the  water  in  the  cistern,  which  must, 


250  On  Heat.  [254- 

therefore,  be  constantly  renewed.  For  this  purpose  a  continual 
supply  of  cold  water  passes  into  the  bottom  of  the  cistern,  while 
the  lighter  heated  water  rises  to  the  surface,  and  escapes  by  a  tube 
in  the  top  of  the  cistern. 

Brandy  is  obtained  from  wine  by  means  of  distillation.  Wine, 
consists  essentially  of  water,  alcohol,  and  colouring  matter  ;  when 
heated  in  a  still  to  a  temperature  between  78°  and  100°,  the  alcohol, 
which  boils  at  78°,  vaporises,  while  water,  which  only  boils  at  100°, 
remains  behind,  or  at  all  events  only  passes  over  in  small  quantity. 
The  liquid  resulting  from  this  distillation,  is  brandy,  which  is  es- 
sentially a  mixture  of  alcohol  and  water. 

255.  Liquefaction  of  gases. — We  have  already  seen  that  a 
saturated  vapour,  the  temperature  of  which  is  constant,  is  lique- 
fied by  increasing  the  pressure,  and  that,  the  pressure  remaining 
constant,  it  is  brought  into  the  liquid  state  by  diminishing  the  tem- 
perature. 

Unsaturated  vapours  behave  in  all  respects  like  gases.  And  it 
is  natural  to  suppose,  that  what  are  ordinarily  called  permanent 
gases  are  really  unsaturated  vapours.  For  the  gaseous  form  is 
accidental,  and  is  not  inherent  in  the  nature  of  the  substance.  At 
ordinary  temperatures  sulphurous  acid  is  a  gas,  while  in  countries 
near  the  poles  it  is  a  liquid  ;  in  temperate  climates  ether  is 
a  liquid,  at  a  tropical  heat  it  is  a  gas.  And  just  as  unsaturated 
vapours  may  be  brought  to  the  state  of  saturation  and  then  liquefied 
by  suitably  diminishing  the  temperature  or  increasing  the  pressure, 
so,  by  the  same  means,  gases  may  be  liquefied.  But  as  they  are 
mostly  very  far  removed  from  this  state  of  saturation  great  cold  and 
pressure  are  required.  Some  of  them  may  indeed  be  liquefied 
either  by  cold  or  by  pressure  ;  for  the  majority,  however,  both 
processes  must  be  simultaneously  employed.  Few  gases  can  resist 
these  combined  actions,  and  probably  those  which  have  not  yet 
been  liquefied,  hydrogen,  oxygen,  nitrogen,  binoxide  of  nitrogen, 
and  carbonic  oxide,  would  become  so  if  submitted  to  a  sufficient 
degree  of  cold  and  pressure. 

One  of  the  most  remarkable  experiments  on  the  liquefaction  of 
gases  is  that  made  by  Thilorier  to  liquefy  and  solidify  carbonic 
acid.  The  principle  of  the  method  was  first  devised  and  applied 
by  Faraday.  The  apparatus  consists  of  two  cast  iron  cylinders 
with  very  thick  sides,  of  5  to  6  quarts  capacity.  They  are  her- 
metically closed,  and  are  connected  by  means  of  a  leaden  tube. 
In  one  of  these  cylinders,  A,  called  the  generator,  are  placed  the 


-255] 


Liquefaction  of  Gases. 


251 


substances  by  whose  chemical  action  carbonic  acid  is  evolved. 
These  are  ordinarily  bicarbonate  of  soda,  D,  and  sulphuric  acid  in 
the  tube,  C.  The  second  cylinder,  called  the  receiver,  B,  is  empty ; 
and  the  gas  disengaged  by  the  chemical  action  in  the  generator 
distils  over,  and  as  the  receiver  is  colder,  it  condenses  in  virtue  of 
its  increasing  pressure.  As  much  as  two  quarts  of  liquid  carbonic 
acid  have  thus  been  prepared. 


Fig.  204. 

At  a  temperature  of  1 5  degrees  the  tension  of  the  compressed 
gas  in  the  cylinders  is  not  less  than  50  atmospheres  ;  a  pressure 
which  would  burst  the  vessels  if  they  were  riot  solidly  constructed. 
An  accident  of  this  kind  happened  some  years  ago,  and  caused  the 
death  of  Thilorier's  assistant. 

To  obtain  solid  carbonic  acid,  the  receiver  is  provided  with  a 
stopcock  attached  to  a  tube,  which  dips  in  the  liquid  acid.  On 
opening  this  stopcock  the  liquid  acid  driven  by  pressure  jets  out ; 
passing  then  from  a  tension  of  50  atmospheres  down  to  a  single 
one,  a  part  of  the  liquid  volatilised  ;  and  in  consequence  of  the 
heat  absorbed  by  this  evaporation,  the  rest  is  so  much  cooled  as 
to  solidify  in  white  flakes  like  snow,  or  anhydrous  phosphoric  acid. 


252  On  Heat.  [255- 

Solid  carbonic  acid  evaporates  very  slowly.  By  means  of  an 
alcohol  thermometer  its  temperature  has  been  found  to  be  about 
—  90°.  A  small  quantity  placed  on  the  hand  does  not  produce  the 
sensation  of  such  great  cold  as  might  be  expected.  This  arises 
from  the  imperfect  contact.  But  if  the  solid  be  mixed  with  ether 
the  cold  produced  is  so  intense,  that  when  a  little  is  placed  on  the 
skin  all  the  effects  of  a  severe  burn  are  produced.  A  mixture  of 
these  two  substances  solidifies  four  times  its  weight  of  mercury  in  a 
few  minutes.  When  a  tube  containing  liquid  carbonic  acid  is 
placed  in  this  mixture  the  liquid  becomes  solid,  and  looks  like  a 
transparent  piece  of  ice. 


CHAPTER   IX. 

SPECIFIC    HEAT.      CALORIMETRY. 

256.  Calorimetry.     Thermal  unit. — The  object  of  calorimetry 
is  to   measure  the  quantity  of  heat  which  a  body  parts  with  or 
absorbs  when  its  temperature  sinks  or  rises  through  a  certain  number 
of  degrees,  or  when  it  changes  its  condition. 

Quantities  of  heat  may  be  expressed  by  any  of  its  directly  mea- 
surable effects,  but  the  most  convenient  is  the  alteration  of  tem- 
perature ;  and  quantities  of  heat  are  usually  defined  by  stating  the 
extent  to  which  they  are  capable  of  raising  the  temperature  of  a 
known  weight  of  a  known  substance,  such  as  water. 

The  unit  chosen  by  comparison,  and  called  the  thermal  unit,  is 
not  everywhere  the  same.  In  France  it  is  the  quantity  of  heat 
necessary  to  raise  the  temperature  of  one  kilogramme  of  water 
through  one  degree  Centigrade  ;  this  is  called  a  calorie.  In  this 
book  we  shall  adopt,  as  a  thermal  unit,  the  quantity  of  heat  neces- 
sary to  raise  one  pound  of  water  through  one  degree  Centigrade  : 
i  calorie  =  2*2  thermal  units,  and  i  thermal  unit  =  0^45  calorie. 

257.  Specific  heat. — When  equal  weights  of  two  different  sub- 
stances at  the  same  temperature  placed  in  similar  vessels  are  sub- 
jected for  the  same  length  of  time  to  the  heat  of  the  same  lamp,  or 
are  placed  at  the  same  distance  in  front  of  the  same  fire,  it  is  found 
that  their  temperature  will  vary  considerably  ;    the  mercury  will 
be  much  hotter  than  the  water.     But  as  from  the  conditions  of  the 


-258]  Determination  of  Specific  Heats.  253 

experiment,  they  have  each  been  receiving  the  same  amount  of 
heat,  it  is  clear  that  the  quantity  of  heat  which  is  sufficient  to 
raise  the  temperature  of  mercury  through  a  certain  number  of  de- 
grees will  only  raise  the  temperature  of  the  same  quantity  of  water 
through  a  less  number  of  degrees  ;  in  other  words,  that  it  requires 
more  heat  to  raise  the  temperature  of  water  through  one  degree 
than  it  does  to  raise  the  temperature  of  mercury  by  the  same  extent. 
Conversely,  if  the  same  quantities  of  water  and  of  mercury  at  100° 
C.  be  allowed  to  cool  down  to  the  temperature  of  the  atmosphere, 
the  water  will  require  a  much  longer  time  for  the  purpose  than  the 
mercury  ;  hence,  in  cooling  through  the  same  number  of  degrees, 
water  gives  out  more  heat  than  does  mercury. 

It  is  readily  seen  that  ail  bodies  have  not  the  same  specific  heat. 
If  a  pound  of  mercury  at  100°  is  mixed  with  a  pound  of  water  at 
zero,  the  temperature  of  the  mixture  will  only  be  about  3°.  That  is 
to  say,  that  while  the  mercury  has  cooled  through  97°,  the  tempe- 
rature of  the  water  has  only  been  raised  3°.  Consequently,  the 
same  weight  of  water  requires  about  32  times  as  much  heat  as 
mercury  does  to  produce  the  same  elevation  of  temperature. 

If  similar  experiments  are  made  with  other  substances  it  will  be 
found  that  the  quantity  of  heat  required  to  effect  a  certain  change  of 
temperature  is  different  for  almost  every  substance,  and  we  speak 
of  the  specific  heat  or  calorific  capacity  of  a  body  as  the  quantity  of 
heat  which  it  absorbs  when  its  temperature  rises  through  a  given 
range  of  temperature,  from  zero  to  i°  for  example,  compared  with 
the  quantity  of  heat  which  would  be  absorbed  under  the  same  cir- 
cumstances, by  the  same  weight  of  water.  In  other  words,  water 
is  taken  as  the  standard  for  the  comparison  of  specific  heats.  Thus, 
to  say  that  the  specific  heat  of  lead  is  0-0314,  means  that  the  quan- 
tity of  heat  which  would  raise  the  temperature  of  any  given  quantity 
of  lead  through  i°  C.  would  only  raise  the  temperature  for  the  same 
quantity  of  water  through  0-0314. 

258.  Determination  of  the  specific  beats  of  solids  and  of 
liquids. — Three  methods  have  been  employed  for  determining  the 
specific  heats  of  bodies  ;  (i.)  the  method  of  melting  ice,  (ii.)  the 
method  of  mixtures,  and  (iii.)  that  of  cooling.  In  the  latter,  the 
specific  heat  of  a  body  is  determined  by  the  time  which  it  takes  to 
cool  through  a  certain  temperature. 

Method  of  the  fusion  of  ice. — This  method  of  determining  specific 
heats  is  based  on  the  fact  that  to  melt  a  pound  of  ice,  80  thermal 
units  are  necessary,  or  more  exactly  79*25.  The  substance  to  be 


254  On  Heat.  [258- 

determined  is  raised  to  a  known  temperature,  100°  for  instance,  and 
is  then  rapidly  placed  in  ice.  In  cooling  from  100°  to  zero,  the 
body  melts  a  certain  quantity  of  ice,  which  is  collected  in  the  form 
of  water.  From  the  weight  of  this  water,  from  that  of  the  body, 
and  from  the  number  of  degrees  through  which  it  is  cooled,  the 
specific  heat  may  be  readily  calculated. 

To  facilitate  the  execution  of  this  method  Lavoisier  and  Laplace 
devised  an  apparatus  which  is  called  the  ice  calorimeter.  Fig.  205 
gives  a  perspective  view  of  it,  and  fig.  206  represents  a  section.  It 
consists  of  three  concentric  tin  vessels,  M,  A,  B,  each  with  covers  of 
the  same  material  ;  in  the  central  one  is  placed  the  body  M,  whose 


Fig.  205. 


Fig.  2o5. 


specific  heat  is  to  be  determined,  while  the  two  others,  A  and  B, 
are  filled  with  pounded  ice.  The  ice  in  the  compartment  A  is 
melted  by  the  heated  body,  and  the  water  resulting  from  the  lique- 
faction runs  off  by  the  stopcock  D,  and  is  collected  in  a  vessel;  the 
ice  in  the  compartment  B  cuts  off  the  heating  influence  of  the  sur- 
rounding atmosphere.  The  stopcock  E  gives  issue  to  the  water 
which  arises  from  the  liquefaction  of  the  ice  in  B. 

Method  of  mixtures. — In  determining  the  specific  heat  of  a  solid 
body  by  this  method,  it  is  weighed  and  raised  to  a  known  tempera- 
ture, by  keeping  it,  for  instance,  for  some  time  in  a  closed  space 
heated  by  steam  ;  it  is  then  immersed  in  a  mass  of  cold  water,  the 
weight  and  temperature  of  which  are  known.  The  water  becomes 


-258] 


Determination  of  Specific  Heats. 


255 


heated  by  the  heat  given  up  by  the  body  in  cooling,  and  both  are 
ultimately  at  the  same  temperature.  From  this  common  tempera- 
ture, from  the  respective  weights  of  the  water  and  of  the  substance, 
and  lastly  from  their  temperatures  at  the  time  of  mixture,  the  spe- 
cific heat  of  the  body  is  deduced  by  a  simple  calculation. 


Specific 

Specific 

Substances. 

heats. 

Substances. 

heats. 

Water    . 

.       I  -0000 

Zinc     v. 

.      0-0955 

Turpentine    . 

.      0-4259 

Copper         . 

.      0-0951 

Wood  charcoal 

.      0-24II 

Silver  .        '", 

.      0-0570 

Sulphur  .         , 

.      0'2025 

Tin     r«       .  .      . 

.      0-0562 

Graphite         . 

.      0'20l8 

Antimony     .      .  . 

.      0-0507 

Thermometer  glass 

.    0-1976 

Mercury 

•      0-0333 

Phosphorus    . 

.   0-1895 

Gold    .         .    :;  . 

•      0-0324 

Diamond 

.    0-1469 

Platinum      .     ••'".' 

.      0-0324 

Iron 

.    0-1138 

Lead    .         .       v 

.      0*0314 

Nickel  . 

.     0-1086 

Bismuth       .         .  . 

.      0-0308 

It  will  be  seen  from  the  above  table  that  water  and  oil  of  tur- 
pentine have  a  much  greater  specific  heat  than  that  of  other  sub- 
stances, and  more  especially  than  the  metals.  It  is  from  its  great 
specific  heat  that  water  requires  a  long  time  in  being  heated  or 
cooled  ;  and  that,  for  the  same  weight  and  temperature,  it  absorbs 
or  gives  out  far  more  heat  than 
other  substances.  This  double 
property  is  applied  in  heating  by 
hot  water,  and  it  plays  a  most  im- 
portant part  in  the  economy  of 
nature. 

Those  bodies  which  have  great 
specific  heat,  and  therefore  which 
require  a  great  quantity  of  heat  to 
raise  them  through  a  given  tempera- 
ture, also  in  cooling  through  the 
same  range  give  out  a  great  quantity. 
This  difference  between  bodies  as  to 
the  quantities  of  heat  they  contain 
may  be  illustrated  by  a  simple  ex- 
periment. A  number  of  small  bullets  of  various  metals,  iron,  tin, 
lead,  bismuth,  and  copper  are  heated  to  a  temperature  of  about 
200°  C  by  immersing  them  in  hot  oil ;  they  are  then  placed  on  a 


Fig.  207. 


256  On  Heat.  [258- 

cake  of  bees-wax,  c  D,  about  half  an  inch  in  thickness  (fig.  207). 
It  will  then  be  found  that  the  iron  and  copper  melt  themselves 
through  first,  then  follows  the  tin,  while  the  lead  and  bismuth 
make  but  little  way,  being  unable  to  sink  much  more  than  half 
their  way  through  the  wax. 


CHAPTER   X. 

STEAM     ENGINES. 

259.  Invention  of  the  steam  engine. — Steam  engines  are  un- 
doubtedly the  most  important  of  the  applications  of  the  physical 
sciences  to  the  arts.  Based  on  the  very  great  elastic  force  which 
aqueous  vapour  assumes  at  a  high  temperature  (247),  and  on  the 
condensation  of  this  vapour  by  cooling  (251),  steam  engines  have 
created,  in  a  small  volume  and  at  small  expense,  very  considerable 
motive  powers. 

Their  importance  has  caused  much  discussion  and  investigation 
as  to  their  inventor,  or  rather  inventors  ;  for  it  is  only  by  the  suc- 
cessive efforts  of  several  men  of  genius  that  these  machines  have  at- 
tained their  present  simplicity  and  precision. 

The  history  of  the  steam  engine  commences  with  Hero,  the  in- 
ventor of  the  fountain  which  bears  his  name,  who  invented,  nearly 
two  thousand  years  ago,  a  steam  tourniquet,  known  as  the  eolipyle, 
analogous  to  the  hydraulic  tourniquet.  The  names  of  Salomon  of 
Caux,  and  then  of  the  Marquis  of  Worcester,  are  mentioned  in  the 
history  of  the  steam  engine. 

Denis  Papin,  a  French  physicist,  to  whom  is  due  the  apparatus 
already  described  (246),  was  the  first  who  caused  a  piston  to 
ascend  in  a  vertical  cylinder  closed  at  the  bottom  and  open  at  the 
top  by  means  of  the  elastic  force  of  steam,  and  to  descend  by  con- 
densing this  vapour  by  cooling ;  so  that  the  piston  which  descended 
in  virtue  of  atmospheric  pressure  had  an  up  and  down  motion  in 
the  cylinder,  which  is  still  the  principle  of  all  steam  engines. 
Papin,  who  was  a  Protestant,  was  obliged  to  fly  from  France  in 
consequence  of  the  revocation  of  the  Edict  of  Nantes,  and  the 
description  and  plan  of  his  machine  was  published  in  Germany  in 
1690.  He  even  made  a  model  large  enough  to  move  a  boat  by 
means  of  paddle-wheels.  In  this  model  there  was  water  under- 


-259] 


Steam  Engines. 


257 


neath  the  piston  at  the  bottom  of  the  cylinder.  When  a  furnace 
was  placed  under  this,  the  water  vaporised,  and  the  elastic  force 
raised  the  piston  ;  when  it  was  at  the  top  of  its  course  the  furnace 
was  withdrawn  ;  the  cylinder  cooling,  the  vapour  was  condensed, 
and  the  piston  sank. 

In  1705  Newcomen  and  Cawley  constructed  a  steam  engine,  or 
'  fire-pump,'  as  it  was  then  called,  the  object  of  which  was  to  drain 
mines.  In  this  engine  the  steam  was  produced  separately  in  a 
boiler  m  below  the  cylinder  c  containing  the  piston^.  The  conden- 


Fig.  208. 

sation  also  was  effected  by  cold  water  from  a  cistern,  n,  being  in- 
jected into  the  cylinder  through  a  cock,  b.  This  was  opened  when 
the  piston  was  to  descend,  and  was  closed  after  the  descent ;  a 
second  one,  a,  was  opened  through  which  steam  entered,  and  so 
on.  But  the  sides  of  the  cylinder  being  cooled  by  this  injection 
of  cold  water,  the  steam  which  filled  it  was  partially  condensed, 
until  the  sides  were  again  heated  :  there  was- thus  a  considerable  loss 
of  steam,  and  therefore  of  fuel.  The  condensed  water  flowed  out 
by  a  pipe,  at  the  end  of  which  was  the  valve  v,  which  opened  as 
the  piston^  descended,  w  w  is  the  beam  by  which  the  motion  is 
transmitted  to  the  pump  rod,  d. 

s 


258  On  Heat.  [260- 

260.  Watt's  improvements  in  the  steam  engine. — James 
Watt,  a  mathematical  instrument  maker  in  Glasgow,  had  to  repair 
the  model  of  a  Newcomen's  engine  belonging  to  the  physical  cabinet 
of  the  University.  Struck  by  the  enormous  quantity  of  steam  and 
of  condensing  water  used  by  this  engine,  he  entered  upon  a  long 
series  of  researches  and  improvements,  which  he  pursued  with  ad- 
mirable perseverance  for  fifty  years,  without  ever  being  content  with 
the  success  he  obtained.  Thus  it  was  that  Newcomen's  machine, 
successively  metamorphosed  in  all  its  parts,  at  last  really  became 
Watt's  machine. 

Condenser.  Watt's  first  and  principal  invention  was  the  condenser. 
This  name  is  given  to  a  closed  vessel  quite  distinct  from  the  cylin- 
der in  which  the  piston  moves,  and  only  connected  with  it  by  a  tube 
provided  with  a  stopcock.  In  this  vessel  cold  water  is  injected, 
and  the  vapour  is  condensed  by  opening  the  connecting  stopcock. 
Thus  as  the  sides  of  the  cylinder  are  not  cooled,  all  the  steam 
which  enters  there  is  utilised.  Thus  there  was  effected  so  great  an 
economy  of  steam,  and  therefore  of  fuel,  that  Watt  and  Boulton 
his  partner,  having  taken  a  patent,  realised  great  profits  by  only 
requiring,  for  a  certain  number  of  years,  a  third  of  the  saving  in 
the  consumption  of  coal  as  compared  with  Newcomen's  engine. 

Single-acting  engine.  In  Newcomen's  engine  the  cylinder  of 
which  was  open  at  the  top,  the  steam  only  lifted  the  piston  ;  and 
then,  when  steam  was  condensed,  the  pressure  of  the  atmosphere 
brought  it  down  again  ;  whence  the  name  atmospheric  engine,  by 
which  it  was  designated.  As  the  piston  descended,  air  penetrated 
into  the  cylinder  and  cooled  the  sides,  in  consequence  of  which  a 
portion  of  the  vapour  which  penetrated  into  the  cylinder  was  con- 
densed until  the  sides  were  again  heated.  To  remove  this  source  of 
loss,  Watt  closed  the  cylinder  altogether,  and  caused  the  vapour  to 
act  above  the  piston,  so  as  to  make  it  descend  ;  then  by  an  arrange- 
ment of  stopcocks,  alternately  opened  and  closed  by  the  action  of 
the  engine  itself,  the  steam  passed  simultaneously  above  and  below 
the  piston.  This  being  pressed  equally  in  opposite  directions,  re- 
mained in  equilibrium  ;  so  that  a  simple  counterpoise  acting  by 
means  of  a  lever  at  the  end  of  the  piston  rod  raised  the  piston 
again,  and  so  on.  This  machine,  into  which  air  did  not  enter,  and 
where  the  atmospheric  pressure  did  not  act,  was  called  the  single- 
acting  engine,  to  express  that  the  steam  had  a  useful  action  on  only 
one  side  of  the  piston. 

The  single-acting  engine  had  the  great  disadvantage  that  it  had 


-260]  Double-acting  Engine.  2. 59 

no  real  force  except  when  the  piston  was  descending.  It  could 
transmit  motion  to  pumps  for  emptying  mines,  because,  for  that, 
effort  in  only  one  direction  was  required  ;  but  it  would  not  furnish 
a  sufficiently  regular  motion  for  many  industries,  for  cotton  manu- 
factures for  instance.  Hence  Watt's  task  was  not  completed  ;  and 
he  was  not  long  in  finding  another  plan. 

Double-acting  engine.  In  this  engine,  one  form  of  which  we 
shall  presently  describe,  and  which  is  represented  in  fig.  209,  the 
cylinder  is  closed  both  at  top  and  at  the  bottom,  but  the  steam  acts 
alternately  on  the  two  faces  of  the  piston  ;  that  is  to  say,  that  by  a 
system  of  stopcocks,  opened  and  closed  by  the  engine  itself,  when 
the  lower  part  of  the  cylinder  communicates  with  the  condenser, 
the  upper  part,  on  the  contrary,  is  connected  with  the  boiler,  and 
the  steam  acting  in  all  its  force  on  the  piston  causes  it  to  descend. 
Then  when  this  is  at  the  bottom  of  its  stroke  the  parts  change  ;  the 
top  of  the  cylinder  is  in  connection  with  the  condenser,  and  the 
bottom  with  the  boiler  ;  the  piston  rises  again  and  so  forth,  whence 
results  an  alternating  rectilinear  motion  which  is  changed  into  a 
continuous  circular  motion,  as  will  be  presently  described  (262). 

Air-pump.  Watt  completed  his  engine  by  the  addition  of  three 
pumps,  which  are  worked  by  the  engine,  and  play  an  important 
part.  For  the  cold  water  of  the  condenser  becomes  rapidly  heated 
by  the  heat  which  the  steam  gives  up  to  it  (253),  and  this  water, 
soon  reaching  100  degrees,  would  no  longer  condense  the  steam. 
Moreover  the  air,  which  is  always  dissolved  in  cold  water,  is  libe- 
rated in  the  boiler,  owing  to  the  increase  in  temperature.  Now  this 
air,  passing  both  above  and  below  the  piston,  would  soon  stop  its 
motion.  To  prevent  these  two  injurious  effects,  Watt  applied 
to  the  engine  a  suction-pump,  which  continually  withdrew  from 
the  condenser  the  air  and  water  which  tended  to  accumulate  there. 

Feed-pump  and  cold-water-piimp.  The  two  other  pumps  which 
Watt  added  are  the  feed-pump  and  the  cold-water-pump.  The 
first  is  a  force-pump  which  sends  into  the  boiler  the  hot  water  with- 
drawn from  the  condenser  by  the  air-pump.,  thnas  producing  a  con- 
siderable saving  in  fuel.  The  other  is  a  suction-pump,  which  raises 
either  from  a  well  or  a  river,  or  some  other  source,  the  cold  water 
intended  to  replace  that  heated  in  the  condenser,  and  withdrawn  by 
the  air-pump. 

Besides  the  important  parts  which  have  thus  been  described,  we 
owe  to  Watt  the  arrangement  for  distributing  the  steam  alternately 
above  and  below  the  piston  :  the  regulator,  whose  function,  when 

S  2 


26o  On  Heat.  [260- 

the  machine  works  too  slowly,  is  to  admit  more  steam  into  the  cy- 
linder, and,  on  the  other  hand,  to  diminish  the  quantity  when  the 
velocity  is  too  great.  Lastly,  the  parallelogram,  which  imparts  to 
the  piston  rod  a  rectilinear  motion.  We  may  add  that  Watt,  who 
had  begun  life  as  a  philosophical  instrument  maker,  carried  into 
the  execution  of  these  great  mechanisms  the  same  perfection  as 
is  required  for  the  best  scientific  instruments. 

261.  Description  of  tbe  double-acting-  engine. — We  have 
already  seen  that  the  double-action  engine  is  that  in  which  the 
steam  acts  alternately  above  and  below  the  piston  (260).  Fig.  209 
represents  an  engine  of  this  kind,  and  fig.  213  gives  a  section  of  the 
cylinder,  of  the  piston,  and  of  the  distribution  of  steam.  The  entire 
engine  is  of  iron.  To  the  piston  T  is  fixed  a  rod  A,  which  slides 
with  gentle  friction  in  a  tubulure  U  placed  at  the  centre  of  the  plate 
which  closes  the  cylinder  (fig.  213).  As  it  is  very  important  that  no 
steam  shall  escape  between  the  piston  rod  and  this  tubulure,  the 
latter  is  formed  of  two  pieces,  one  attached  to  the  plate,  while  the 
other,  which  fits  in  the  first,  can  be  pressed  as  tightly  as  is  desired, 
so  as  to  compress  the  material  soaked  with  fat  which  is  between 
the  two  tubulures.  This  arrangement  is  called  a  stuffing  box ;  it 
prevents  the  escape  of  steam  without  interfering  with  the  motion  of 
the  piston. 

On  the  two  sides  of  the  cylinder  are  two  columns  h  k,  which 
guide  the  piston  rod  in  its  upward  and  downward  motion.  The  end 
of  the  piston  rod  is  connected  with  a  long  piece  B,  called  the  con- 
necting rod,  which  in  turn  is  jointed  with  a  shorter  piece  M,  called 
the  crank,  the  length  of  which  is  just  half  that  of  the  stroke  of  the 
piston.  This  is  rigidly  fixed  to  a  horizontal  shaft,  D,  so  that  it  can- 
not move  without  transmitting  its  motion. 

By  means  of  this  connecting  rod  and  crank,  the  alternating  rec- 
tilinear motion  of  the  piston  and  of  the  rod  is  changed  into  a  con- 
tinuous circular  motion.  For  the.rod  during  the  ascent  of  the  piston, 
acts  upwards  upon  the  crank,  making  it  turn  in  the  direction  of  the 
arrow.  When  the  piston  is  at  the  top  of  its  stroke,  the  motion  rod 
and  the  crank  are  one  in  front  of  the  other.  As  the  piston  descends 
the  motion  rod  again  acts,  so  as  always  to  turn  it  in  the  same  direc- 
tion ;  and  when  the  piston  is  at  the  bottom  of  the  stroke,  they  are 
again  vertical,  but  one  in  the  prolongation  of  the  other.  Hence  it 
follows  that  the  axle  which  has  made  half  a  turn  during  the  ascent, 
makes  a  second  one  during  the  descent,  and  thus  a  complete  revo- 
lution during  each  double  oscillation  of  the  piston. 


-261] 


Double-acting  Engine, 


261 


To  transmit  the  motion  to  machinery,  on  the  axle  D  is  fixed  a 
sheave  on  which  works  an  endless  band  XY  of  leather,  or  of  gutta- 


"K*W 


Fig.  209. 


262  On  Heat.  [261- 

percha,  which  works  on  another  sheave  fixed  to  the  machinery  to 
be  turned.  Moved  by  the  first  sheave,  this  band  communicates  its 
motion  to  the  second  ;  in  this  manner  the  motion  is  transmitted  to 
all  the  workshops  of  a  large  factory.  On  the  right  of  the  fixed 
sheave,  G,  there  is  a  second,  which  is  not  fixed  to  the  horizontal 
shaft ;  this  is  the  movable  sheave.  Its  object  is  to  suspend  all  the 
motion  in  the  machine  without  stopping  the  steam  engine.  By 
means  of  an  iron  fork  not  seen  in  the  figure,  which  encloses  the 
band,  the  latter  may  be  slid  from  the  fixed  to  the  movable  sheave.  As 
this  latter  is  not  connected  with  the  horizontal  shaft,  it  does  not 
turn  with  it,  and  does  not  transmit  its  motion  to  the  band. 

On  the  horizontal  shaft  is  a  very  large  iron  wheel  V,  called  the 
fly-wheel,  which  is  necessary  for  keeping  up  the  motion.  For  each 
time  that  the  piston  is  at  the  top  or  bottom  of  its  stroke,  there  is  a 
momentary  arrest,  during  which  the  motion  of  the  whole  machine 
tends  to  stop.  These  are  called  the  dead  points.  It  is  then 
that  the  fly-wheel,  in  virtue  of  its  inertia  and  of  its  acquired 
velocity,  moves  the  horizontal  shaft,  and  thus  keeps  up  a  regular 
motion. 

262.  Excentric.  Valve-chest. — The  excentric  is  an  arrange- 
ment by  which  a  continuous  circular  motion  is  changed  into  an 
alternating  rectilinear  motion.  It  is  very  frequently  used  in 
machinery. 

One  of  these  is  fitted  to  the  horizontal  shaft  at  E,  and  the  other 
at  e.  The  former  works  the  feed-pump,  and  the  latter  the  valve- 
chest.  The  action  of  both  is  the  same.  Figures  210  and  211  repre- 
sent it  on  a  larger  scale,  in  two  diametrically  opposite  positions. 
It  consists  of  a  circular  piece  KE,  fixed  to  the  horizontal  shaft,  but 
in  such  a  manner  that  the  centre  of  rotation  does  not  coincide  with 
the  centre  of  the  piece  ;  the  latter  being  at  C,  the  former  at  O.  It 
follows  from  this  construction  that  the  point  C  constantly  describes 
a  circumference  about  O,  which  is  represented  in  the  drawing  by  a 
dotted  line.  Hence  in 'each  half  turn  it  passes  from  the  position 
represented  in  fig.  210  to  that  represented  in  fig.  211,  and  vice  versa. 
So  that  the  point  C,  in  turning  about  the  point  O,  does  really  per- 
form an  up  and  down  motion. 

To  use  this  motion,  the  excentric  is  surrounded  by  a  collar  run, 
in  which  it  can  turn  freely  like  an  axle  in  its  box  :  hence,  during 
the  rotation  of  the  horizontal  shaft,  the  collar  shares  the  ascend- 
ing and  descending  motion  of  the  point  C,  but  not  its  rotatory 
motion.  The  excentric  alone  turns,  the  collar  only  rises  a.nd 


-262] 


Valve-chest. 


263 


sinks.      By  thus  transmitting  its  motion  to  a  rod  /,  it  works  the 
valve-^ehest. 


Fig.  210. 


Fig.  2ii. 


Valve-chest.  We  have  still  to  describe  the  valve-chest,  the  ar- 
rangement by  which  steam  passes  alternately  above  and  below  the 
piston.  Fig.  213  presents  a  vertical  section  of  this  valve-chest,  and 
of  the  cylinder.  The  steam  enters  the  valve-chest  from  the  boiler 
by  the  brass  tube  x.  From  the  valve-chest  two  conduits,  a  and  ^, 
are  connected  with  the  cylinder,  one  above  and  the  other  below. 
If  they  were  both  open  at  once,  the  steam  acting  equally  on  the  two 
faces  of  the  piston  would  keep  it  at  rest.  But  one  of  these  is 
always  closed  by  a  slide  valve,  y,  fixed  to  a  rod,  /.  This  moves 
alternately,  up  and  down,  by  means  of  an  excentric,  e,  placed 
on  the  horizontal  shaft.  In  fig.  213  the  slide-valve  closes  the  con- 
duit «,  and  allowing  the  steam  to  enter  at  b,  below  the  piston,  the 
latter  rises.  But  when  it  reaches  the  top  of  the  stroke  the  excentric 
has  passed  from  the  position  represented  in  fig.  210  to  that  in  fig. 
21 1  ;  hence  the  rod,  /,  sinks,  and  with  it  the  slide-valve,  which  then 
closes  the  conduit  b,  and  allows  the  vapour  to  enter  at  a  (fig. 
212).  The  piston  then  sinks,  and  so  forth  at  each  displacement  of 
the  slide  valve. 

In  completing  this  account  of  the  manner  in  which  steam  is  dis- 


264 


On  Heat. 


[262- 


tributed,  it  remains  to  explain  what  happens  when  the  steam  presses 
below  the  piston  (fig.  213).  It  must  not  remain  above,  otherwise 
the  piston  could  not  move.  But  while  the  steam  enters  below  by 
the  conduit  b,  the  top  of  the  cylinder,  by  means  of  a  conduit  a,  is 
connected  with  a  cavity  O,  from  which  passes  a  tube  L.  Through 
this  tube  the  steam  which  has  already  acted  upon  the  piston  passes 
into  the  atmosphere,  or  else  is  condensed  in  a  vessel  filled  with 


Fig. 


Fig.  213. 


cold  water,  which  has  been  already  mentioned,  the  condenser  (262). 
If,  on  the  other  hand,  the  piston  sinks,  the  slide-valve  being  in 
the  position  of  fig.  212,  the  vapour  below  the  piston  passes  by  the 
conduit  &,  to  the  cavity  O,  and  to  the  tube  L. 

263.  Regulator. — The  object  of  this  arrangement  is  to  regulate 
the  quantity  of  steam  which  reaches  the  valve-chest,  increasing  it 
when  the  machine  works  too  slowly  and  diminishing  it  when  it 
works  too  rapidly.  It  consists  of  a  parallelogram  kr,  each  apex  of 


-265]  Various  Kinds  of  Steam  Engines.  26$ 

which  is  jointed.  A  toothed  wheel,  a,  connected  with  the  horizontal 
shaft,  transmits  its  motion  to  a  similar  wheel,  b,  fixed  to  the  rod  c, 
which  supports  the  parallelogram.  This  turns  then  with  the  rod 
the  more  rapidly  the  greater  the  velocity  of  the  machine.  But  the 
two  upper  arms  are  provided  with  two  solid  balls,  m  and  n ; 
moreover,  a  socket,  r,  to  which  are  attached  the  two  lower  arms,  is 
not  fixed  to  the  rod  c,  but  can  glide  along  it.  Hence  the  centri- 
fugal force  (29)  acting  on  the  balls  m  and  n  makes  them  diverge, 
the  parallelogram  opens,  and  the  socket  rises.  It  transmits  its 
motion  to  a  lever,  s,  the  short  arm  of  which  being  lowered  presses 
upon  a  long  rod,  /.  This  inclining  the  lever,  O,  effects  a  small  rota- 
tion in  a  valve,  v,  placed  in  the  tube  .r,  by  which  steam  comes 
(fig.  213).  This  valve,  either  by  stopping  the  tube  x,  or  leaving  it 
open,  admits  more  or  less  steam. 

264.  Feed-pump. — The  object  of  this,  as  its  name  implies,  is  to 
renew  the  water  in  the  boiler  in  the  degree  in  which  it  evaporates. 
In  fig.  209  this  pump,  placed  at  Q,  on  the  left  of  the  drawing,  re- 
ceives its  motion  from  an  excentric  by  means  of  a  long  rod,  and  it 
works  both  as  cold-water-pump  and  as  feed-pump  ;  as  cold-water- 
pump,  inasmuch  as  it  withdraws  water  from  a  well  by  a  suction-pipe 
placed  below  the  engine  ;  and  as  feed-pump  by  its  then  forcing 
water  into  the  boiler  by  the  pipe  R. 

265.  Various  kinds  of  steam  engines. — A  low  pressure  engine 
is  one  in  which  the  pressure  of  the  vapour  does  not  much  exceed  an 
atmosphere  ;   and  a  high  pressure  engine   is   one   in   which   the 
pressure  of  the  steam  usually  exceeds  this  amount  considerably. 
Low  pressure  engines  are  mostly  condensing  engines  :    in   other 
words,  they  generally  have  a  condenser  where  the  steam  becomes 
condensed  after  having  acted  on  the  piston  ;  on  the  other  hand, 
high  pressure  engines  are  frequently  without  a  condenser ;  the  loco- 
motive is  an  example. 

If  the  communication  between  the  cylinder  and  the  boiler  remains 
open  during  the  whole  motion  of  the  piston,  the  steam  retains  es- 
sentially the  same  elastic  force,  and  is  said  to  act  without  expansion  : 
but  if,  by  a  suitable  arrangement  of  the  slide-valve,  the  steam  ceases 
to  pass  into  the  cylinder  when  the  piston  is  at  |  or  f  of  its  course, 
then  the  vapour  expands  ;  that  is  to  say,  in  virtue  of  its  elastic  force 
which  is  due  to  the  high  temperature,  it  still  acts  on  the  piston  and 
causes  it  to  finish  its  course.  Hence  a  distinction  is  made  between 
expanding  and  non-expanding  engines. 

The  principle  of  expansion  is  not  applicable  to  low  pressure 


266 


On  Heat. 


[265- 


engines,  for  the  elastic  force  of  the  steam  is  inconsiderable.  But 
for  high  or  mean  pressure  engines  it  not  only  effects  a  great  saving 
in  steam,  and  therefore  in  fuel,  but  it  regulates  the  motion  by 
diminishing  the  pressure  the  moment  the  acquired  velocity  of  the 
piston  tends  to  increase. 

266.  Work  of  an  engine.  Horse-power. — The  work  of  an 
engine  is  measured  by  the  mean  pressure  on  the  piston  multiplied 
by  the  area  of  the  piston  multiplied  by  the  length  of  the  stroke.  In 


Fig.  214. 

England  the  unit  of  work  is  the  foot-pound  ;  that  is,  the  work  per- 
formed in  raising  a  weight  of  one  pound  through  a  height  of  a  foot. 
Thus,  to  raise  a  weight  of  14  pounds  through  a  height  of  20  feet 
would  require  280  foot-pounds.  In  France  the  kilogrammetre  is 
used  ;  that  is,  the  work  performed  in  raising  a  kilogramme  through 
a  metre.  This  unit  corresponds  to  7*233  foot-pounds. 

The  rate  of  work  in  machines  is  the  amount  of  work  performed 


-267] 


Steam  Boiler. 


in  a  given  time  ;  a  second  or  an  hour,  for  example.  In  England 
the  rates  of  work  are  compared  by  means  of  horse-power,  which  is 
a  conventional  unit,  and  represents  550  foot-pounds  in  a  second. 
In  France  a  similar  unit  is  used,  called  the  cheval  vapeur,  which 
represents  the  work  performed  in  raising  75  kilogrammes  through 
one  metre  in  a  second.  It  is  equal  to  about  542  foot-pounds  per 
second. 

267.  Steam  boiler. — We  have  still  to  describe  the  steam  boiler 
or  arrangement  by  which  the  steam  is  generated,  and  its  various 
accessories. 

Fig.  214  gives  a  longi- 
tudinal and  fig.  215  a  trans- 
verse section  of  the  steam 
boiler  and  its  furnace.  The 
generator  consists  of  two 
wrought  iron  cylinders  with 
hemispherical  ends.  Below 
are  two  cylinders,  BB,  of 
smaller  diameter,  which  are 
called  heaters,  and  which  are 
connected  with  the  generators 
by  two  strong  tubes.  The 
object  of  these  heaters  is  to 
expose  a  greater  surface  to 
be  heated.  They  are  full  of 
water,  as  also  are  the  tubes 
which  connect  them  with  the 
boiler,  which  is  only  half  full. 

The  feed-water  sent  by  the  pump,  Q,  reaches  the  boiler  by  a  tubu- 
lure,  11,  which  is  immersed  to  the  bottom  to  prevent  cold  water  from 
condensing  steam  ;  a  second  tubulure, m,  leads,  the  vapour  to  the 
valve-chest.  In  the  middle  of  the  boiler  is  an  oval  hole,  called  a 
manhole,  the  object  of  which  is  to  allow  workmen  to  enter  the 
boiler  when  it  needs  repair.  This  hole,  as  well  as  two  front  ones, 
B  B,  of  the  heaters  are  closed  by  what  are  called  autoclaves.  Here 
the  cover  instead  of  being  on  the  outside  is  on  the  inside.  A  screw 
T  fixed  to  this  cover  makes  it  press  against  the  sides  ;  and  as  the 
pressure  of  the  steam  acts  in  the  same  direction,  the  greater  the 
pressure  the  more  tightly  is  the  vessel  closed. 

The  furnace  in  which  the  boiler  is  placed,  is  so  constructed  as 
to  multiply  the  surface  heated,  and  to  render  the  combustion  as 


Fig.  215. 


268  On  Heat.  [267- 

complete  as  possible.  The  products  of  combustion  pass  into  tall 
chimneys,  which  from  their  great  height  increase  the  draught  and 
thereby  promote  the  combustion. 

268.  Float. — This  is  a  small  apparatus,  the  object  of  which  is 
to  show  the  level  of  water  in  the  boiler.     It  consists  of  a  lever, 
at  one  end  of  which  is  a  piece  of  stone,-  F,  and  at  the  other  a  counter- 
poise, a.      The  mass  F  weighs  more  than  the  counterpoise  a ;  but 
as  it  is  immersed  in  water,  and  thus  loses  part  of  its  weight  (96),  it 
is  in  equilibrium,  and  the  lever  is  horizontal  so  long  as  the  level  of 
water  is  at  the  desired   height.     But  it  sinks  when  there  is  too 
little  water,  and  rises  in  the  contrary  direction  when  there  is  too 
much.     Guided  by  these  indications,  the  stoker  can  regulate  the 
supply  of  water. 

269.  Safety-valve. — The   pressure  of  steam  in  the  boiler   is 
measured  by  means  of  the  manometer  (134).     But  this  instrument 

would  not  prevent  explosions  if 
its  indications  were  neglected. 
Hence  on  boilers  two  safety- 
valves,  are  placed,  similar  to  that 
which  Papin  adopted  in  his  di- 
gester (248).  Fig.  216  rep  resents 
on  a  larger  scale  one  of  these 
valves.  It  consists  of  a  metal 
stopper  c  closing  a  tubulure  A, 
fixed  on  the  boiler.  To  prevent 
Flg-  2l6<  this  from  sticking  to  the  sides, 

the  metal  stopper  is  hollowed  on  three  sides,  as  shown  at  S.  It  thus 
more  resembles  a  clack-valve  than  an  ordinary  cork.  On  the  piece 
rests  a  movable  lever,  ab,  loaded  with  a  weight,/.  By  moving  this 
along  the  lever  the  load  on  the  valve  can  be  modified  at  will.  For 
this  purpose  marks  are  placed  which  indicate  the  position  of  the 
load  which  corresponds  to  a  given  pressure.  Thus,  suppose  it  is 
desired  that  the  pressure  shall  not  exceed  5  atmospheres,  the 
weight  is  placed  at  the  division  5  on  the  lever.  Then,  as  long  as 
the  pressure  is  less  than  5,  the  safety-valve  remains  closed  :  but, 
if  the  pressure  exceeds  this  amount,  the  valve  opens  and  gives  exit 
to  the  steam,  thus  preventing  an  explosion. 

270.  Safety-whistle. — This  is  another  safety  apparatus,  which 
indicates  at  a  distance  when  the  level  of  water  in  the  boiler  is  too 
low.     It  consists  of  a  float,  F  (fig.  217),  supported  by  a  lever,  ih, 


-270] 


Safety  Whistle. 


269 


which  moves  about  the  joint  c  ;  a  counterpoise,  ^,  balances  the 
float,  and  a  small  conical  stopper,  a,  fixed  to  the  lever,  closes  a 
tubulure  on  the  boiler.  This  tubulure  is  closed  at  the  top  by  two 
hollow  hemispheres.  In  the  centre  of  the  lower  arc  is  a  disc,  which 
does  not  quite  reach  the  edges.  Between  the  two  hemispheres  is  a 
circular  interval  through  which  vapour  escapes  when  the  cone  a 
does  not  close  the  tubulure. 

As  long  as  the  water  is  at  the  right  height  the  float  F  is  raised, 


Fig.  217. 

and  presses  the  cone  against  the  tubulure  ;  but  if  the  level  sinks 
the  float  sinks,  and  with  it  the  cone.  The  steam  escapes  round  the 
disc  e,  and  gives  a  very  acute  sound  in  striking  against  the  edges 
of  the  upper  hemisphere  which  are  bevelled.  The  system  con- 
stitutes, in  fact,  an  organ  pipe  with  a  very  short  mouthpiece,  and 
yielding,  therefore,  a  very  acute  sound. 

On  locomotives  a  similar  whistle  enables  the  driver  to  signal  at 
a  great  distance  by  opening  a  stopcock,  which  allows  the  steam  to 
escape. 


270  On  Heat.  [271- 


CHAPTER  XI. 

HYGROMETRY. 

271.  Object  of  hygrometry. — The  object  of  hygrometry  is  to 
determine  the  quantity  of  aqueous  vapour  contained  in  a  given 
volume  of  air.     This  quantity  is  very  variable  ;  but  the  atmosphere 
is  never  completely  saturated  with  vapour,  at    any  rate,  in   our 
climates.      Nor  is  it  ever  completely  dry  ;  for  if  hygrometric  sub- 
stances, that  is  to  say,  substances  with  a  great  affinity  for  water, 
such  as  chloride  of  calcium,  sulphuric  acid,  etc.,  be  at  any  time 
exposed  to  the  air,  they  absorb  more  or  less  aqueous  vapour. 

The  degree  of  moisture  does  not  depend  on  the  absolute  quantity 
of  aqueous  vapour  present  in  the  air,  but  on  the  greater  or  less 
distance  of  the  air  from  its  point  of  saturation.  When  the  air  is 
cold,  it  may  be  moist  with  very  little  vapour,  and,  on  the  contrary, 
when  it  is  warm,  it  may  be  very  dry,  even  with  a  large  quantity  of 
vapour.  In  summer  the  air  usually  contains  more  aqueous  vapour 
than  in  winter,  notwithstanding  which  it  is  less  moist,  because, 
the  temperature  being  higher,  the  vapour  is  farther  from  its  point 
of  saturation.  When  a  room  is  warmed,  the  quantity  of  moisture  is 
not  diminished,  but  the  moisture  of  the  air  is  lessened,  because  its 
point  of  saturation  is  raised.  The  air  may  thus  become  so  dry  as 
to  be  injurious  to  the  health,  and  it  is  hence  usual  to  place  vessels 
of  water  on  the  stoves  used  for  heating. 

The  quantity  of  vapour  contained  in  the  air  varies  greatly  with 
the  seasons,  the  climates,  the  temperature,  and  various  local  causes. 
A  mean  degree  of  moisture  is  best  suited  to  the  animal  economy. 
In  a  state  of  great  dry  ness,  as  is  the  case,  for  instance,  during  the 
prevalence  of  north-east  winds,  the  cutaneous  transpiration  is  too 
abundant,  the  skin  dries  up  and  chaps,  and  general  discomfort 
ensues.  In  an  atmosphere  which  is  too  moist,  perspiration  stops,  a 
feeling  of  depression  and  heaviness  is  felt.  Hence  it  is  necessary 
to  regulate  in  a  suitable  manner  the  moisture  of  dwelling  rooms, 
so  as  to  avoid  these  two  extremes. 

272.  Hygroscopes  and  hygrometers. — There  are  two   classes 
of  instruments  by  which  the  hygrometric  state  of  the  air  may  be 


-273] 


Hygrometric  State  of  the  A  ir. 


271 


known.  One  class,  called  hygroscopes,  simply  tell  whether  the  air 
is  more  or  less  moist,  but  give  no  indications  as  to  the  quantity  of 
moisture  it  contains  ;  others,  called  hygrometers,  enable  us  to  mea- 
sure it  with  some  accuracy. 

All  substances  which  absorb  aqueous  vapour,  like  common  salt 
and  many  others  known  as  deliquescent  salts,  may  serve  as  hygro- 
scopes.  This  is  also  the  case 
with  a  great  number  of  animal 
and  vegetable  substances,  such 
as  paper,  parchment,  hair,  cat- 
gut, etc.,  which,  elongating 
as  the  air  becomes  moist,  and 
contracting  as  it  becomes  dry, 
give  an  indication  of  the  greater 
or  less  quantity  of  vapour  in  the 
air. 

A  great  number  of  instru- 
ments have  been  constructed 
which  serve  as  hygroscopes.  One 
of  the  commonest  is  that  repre- 
sented in  fig.  218.  It  consists  of 
a  small  figure  representing  a 
monk  fixed  on  a  support  ;  the 
head  is  provided  with  a  cowl  of 
thin  cardboard,  movable  about 
the  point  a,  where  it  is  attached 

to  the  end  of  a  small  piece  of  Fig-  2l8- 

twisted  catgut.  The  other  end  of  this  is  fixed  in  a  tubulure,  o,  as 
seen  in  the  section.  The  catgut  twisting  as  it  becomes  dry,  and 
untwisting  as  it  is  moist,  moves  the  cowl  which  is  carefully  arranged, 
so  that  the  head  is  -covered  when  the  atmosphere  is  moist,  and  un- 
covered when  it  is  dry. 

This  instrument,  and  all  others  of  the  same  class,  only  change 
slowly,  and  their  indications  are  always  behindhand  with  the  state 
of  the  weather ;  nor  are  they,  moreover,  very  exact. 

273.  Hygrometric  state  of  the  air. — By  this  term  we  do  not 
understand  the  actual  quantity  of  vapour  present,  but  the  ratio  of 
the  quantity  of  vapour  which  the  air  actually  contains  to  that  which 
it  would  contain  if  it  were  saturated.  Thus,  if  we  say  that  the  air 
is  three-fifths  saturated,  we  mean  that  it  contains  three-fifths  of  the 
vapour  which  it  would  contain  in  a  state  of  saturation. 


2/2 


On  Heat. 


[273- 


The  most  exact  of  all  hygrometers  are  the  chemical  hygrometers. 
They  consist  essentially  of  an  arrangement  by  which  a  given 
measured  volume  of  air  is  passed  through  a  series  of  drying  tubes 
— that  is,  tubes  containing  some  hygroscopic  substance,  such  as 
chloride  of  calcium,  or  pumice  saturated  with  sulphuric  acid.  These 
tubes,  being  previously  weighed,  are  weighed  again  after  the 
operation ;  an  increase  of  weight  is  observed,  which  is  due  to  the 
moisture  absorbed  by  the  hygroscopic  substance,  and  this  increase 
represents  the  weight  of  the  moisture  in  the  volume  of  air  taken. 

This  method  is  very  exact,  but  it  is  both  difficult  and  tedious  of 
execution. 

More  convenient  than  the  above  are  what  are  called  condensation 
hygrometers,  in  which  the  vapour  of  the  atmosphere  is  made  to  con- 
dense on  a  body  artificially  cooled.  When  a  body  gradually  cools 

in  a  moist  atmosphere,  the  layer 
of  air  in  immediate  contact  with 
it  cools  also,  and  a  point  is  ulti- 
mately reached  at  which  the  va- 
pour present  is  just  sufficient  to 
saturate  the  air  :  the  least  dimi- 
nution of  temperature  then  cau- 
ses a  precipitation  of  moisture  on 
the  body  in  the  form  of  dew. 
When  the  temperature  rises 


again,  the  dew  disappears,  and 
the  mean  of  these  two  tempera- 
tures is  taken  as  the  dew  point. 
A  good  example  of  an  instru- 
ment of  this  class  is  met  with 
in  DanielPs  hygrometer.  This 
consists  of  two  glass  bulbs  at 
the  extremities  of  a  glass  tube 
bent  twice  (fig.  219).  The  bulb 
A  is  two-thirds  full  of  ether, 
Fig  2I9  and  a  very  delicate  thermo- 

meter dips   in   it ;   the   rest   of 

the  space  contains  nothing  but  the  vapour  of  ether,  the  ether 
having  been  boiled  before  the  bulb  B  was  sealed  The  bulb 
B  is  covered  with  muslin,  and  ether  is  dropped  upon  it.  The  ether 
in  evaporating  cools  the  bulb,  and  the  vapour  contained  in  it  is  con- 
densed. The  internal  tension  being  thus  diminished,  the  ether  in 


-273] 


Wet  Bulb  Hygrometer. 


273 


A  forms  vapours  which  condense  in  the  other  bulb,  B.  In  propor- 
tion as  ether  distils  from  the  lower  to  the  upper  bulb,  the  ether  in  A 
becomes  colder,  and  ultimately  the  temperature  of  the  air  in  imme- 
diate contact  with  A  sinks  to  that  point  at  which  its  vapour  is  just 
more  than  sufficient  to  saturate  it,  and  the  excess  is  accordingly 
deposited  on  the  outside  as  a  ring  of  dew  corresponding  to  the  sur- 
face of  the  ether.  The  temperature  of  this  point  is  noted  by  means 
of  the  thermometer  in  the  inside.  The  addition  of  ether  to  the  bulb 
B  is  then  discontinued,  the  temperature  of  A  rises, 
and  the  temperature  at  which  the  dew  disappears 
is  noted.  In  order  to  render  the  deposition  of 
dew  more  perceptible,  the  bulb  A  is  made  of  black 
glass. 

These  two  points  having  been  determined,  their 
mean  is  taken  as  that  of  the  dew-point.  The  tem- 
perature of  air  at  the  time  of  the  experiment  is 
indicated  by  the  thermometer  on  the  stem.  The 
tension  f,  corresponding  to  the  temperature  of  the 
dew  point,  is  then  found  in  the  table  of  tensions  (249). 
This  tension  is  exactly  that  of  the  vapour  present  in 
the  air  at  the  time  of  the  experiment.  The  tension, 
F  of  vapour  saturated  at  the  temperature  of  the  at- 
mosphere is  found  by  means  of  the  same  table  ;  the 
quotient  obtained  by  dividing  /  by  F,  represents 
the  hygrometric  state  of  the  air.  For  instance,  the 
temperature  of  the  air  being  15°,  suppose  the  dew 
point  is  5°.  From  this  table  the  corresponding  ten- 
sions are  /=  6-5 3  millimetres,  and  F  =  i27o  milli- 
metres, which  gives  0-514  for  the  ratio  of /to  F,  or 
the  hygrometric  state. 

A  very  convenient  form  of  hygrometer,  and  one 
whose  use  is  gradually  extending,  is  that  known  as  the  Pyschrometer 
or  wet  bulb  hygrometer,  which  is  based  on  the  principle  that  a 
moistened  body  evaporates  in  the  air  more  rapidly  in  proportion 
as  the  air  is  drier  (242) ;  and,  in  consequence  of  this  evaporation, 
the  temperature  of  the  body  sinks.  The  application  of  the  prin- 
ciple to  this  purpose  was  first  suggested  by  Leslie.  The  form  of 
the  apparatus  usually  adopted  in  this  country  is  due  to  Mason.  It 
consists  of  two  delicate  thermometers  placed  on  a  wooden  stand 
(fig.  220).  One  of  the  bulbs  is  covered  with  muslin,  and  is  kept 
continually  moist  by  being  connected  with  a  reservoir  of  water  by 

T 


Fig.  220. 


2/4 


On  Heat. 


[273- 


means  of  a  string.  Unless  the  air  is  saturated  with  moisture,  the  wet 
bulb  thermometer  always  indicates  a  lower  temperature  than  the 
other,  and  the  difference  between  the  indications  of  the  two  thermo- 
meters is  greater  in  proportion  as  the  air  can  take  up  more  moisture. 
According  to  Glashier  the  temperature  of  the  dew  point  may  be 
obtained  by  multiplying  the  difference  between  the  temperatures  of 
the  wet  and  dry  bulb  by  a  number  which  depends  on  the  temperature 
of  the  air  at  the  time  of  observation,  and  subtracting  the  product 
thus  obtained  from  this  last-named  temperature.  The  following 
are  the  numbers  : 


Dry  bulb 
temperature  F.° 

Factor 

Dry  bulb 
temperature  F.° 

Factor 

Below  24° 

8-5 

34  to  35° 

2-6 

24t025 

7'3 

35—40 

2'5 

25  —  26 

6-4 

40—45 

2-3 

26  —  27 

6-1 

45—50 

2'I 

27—28 

5'9 

50—55 

2'0 

28—29 

57 

55—60 

r8 

29—30 

5-0 

60—65 

1-8 

30-31 

4-6 

65—70 

17 

31—32 

3-6 

70-75 

1-5 

32—33 

3'i 

75—80 

1-3 

33—34 

2-8 

80—85 

fO 

These  are  often  known  as  Glashier 's  factors.    The  temperatures 
are  expressed  on  the  Fahrenheit  scale. 


CHAPTER  XII. 

METEOROLOGICAL   PHENOMENA  WHICH   DEPEND   UPON   HEAT. 

274.  Meteorology. — Meteorology  is  that  part  of  physics  which 
is  concerned  with  the  phenomena  which  occur  in  the  atmosphere  ; 
such,  for  instance,  as  variations  in  the  temperature  of  the  air,  wind, 
rain,  storms,  electrical  phenomena,  etc.  Though  of  quite  recent 
origin,  this  science  is  an  important  application  of  the  physical 
sciences,  and  furnishes  useful  indications  to  navigation,  to  agricul- 
ture, and  to  hygiene. 


-276]  Mean  Temperature.  275 

275.  Mean  temperature. — The  mean  daily  temperature,  or 
simply  temperature,  is  that  obtained  by  adding  together  24  hourly 
observations,  and  dividing  by  24.  A  very  close  approximation  to 
the  mean  temperature  is  obtained  by  taking  the  mean  of  the 
maxima  and  minima  temperatures  of  the  day  and  of  the  night, 
which  are  determined  by  means  of  the  maximum  and  minimum 
thermometers  (204).  These  ought  to  be  protected  from  the  solar 
rays,  raised  above  the  ground,  and  be  far  from  all  objects  which 
might  influence  them  by  their  radiation.  The  lowest  daily  tem- 
perature is  at  4  A.M.,  and  the  highest  at  2  P.M. 

The  temperature  of  a  month  is  the  mean  of  those  of  30  days,  and 
the  temperature  of  the  year  is  the  mean  of  those  of  12  months.  The 
highest  mean  monthly  temperature  is  in  July,  and  the  lowest  in 
January.  Finally,  the  temperature  of  a  place  is  the  mean  of  its 
annual  temperature,  for  a  great  series  of  years.  The  mean  tem- 
perature of  London  is  8*28°  C.,  or  46*9°  F.  The  temperatures  in 
all  cases  are  those  of  the  air  and  not  those  of  the  ground. 

276.  Causes  which  modify  the  temperature  of  the  air. — The 
principal  causes  which  modify  the  temperature  of  the  air  are  the 
latitude  of  a  place,  its  height — that  is,  its  distance  above  the  sea — 
the  direction  of  the  winds,  and  the  proximity  of  seas. 

Influence  of  the  latitude.  The  temperature  of  the  air  and  of  the 
ground  diminishes  from  the  equator  towards  the  poles.  This  is 
due  to  the  fact,  that  the  sun's  rays,  which  are  perpendicular  at  the 
equator,  are  more  and  more  inclined  as  we  come  near  the  poles. 
Now  we  have  seen  (215)  that  the  greater  the  obliquity  under  which 
the  rays  of  heat  fall  upon  a  body,  the  less  is  the  body  heated  ; 
hence  the  heat  absorbed  decreases  from  the  equator  to  the  poles,  for 
the  rays  are  then  more  oblique.  Yet,  as  in  summer,  the  days  are 
longer  as  we  get  nearer  the  north,  the  loss  due  to  the  increasing 
obliquity  of  the  sun  is  partially  compensated  by  the  sun  remaining 
longer  above  the  horizon.  Under  the  equator,  where  the  length 
of  the  days  is  constant,  the  temperature  is  almost  invariable  ;  in 
the  latitude  of  London,  and  the  more  northerly  countries,  where  the 
days  are  very  unequal,  the  temperature  varies  greatly  ;  but  in 
summer  it  sometimes  rises  almost  as  high  as  under  the  equator. 
The^lowering  of  the  temperature  produced  by  the  latitude  is  small  ; 
thus  in  a  latitude  of  115  miles  north  of  ours,  the  temperature  is 
only  i°  C.  lower. 

Influence  of  altitude.  The  height  of  a  place  has  a  much  more 
considerable  influence  on  the  temperature  than  its  latitude.  In 

T  2 


2;6  On  Heat.  [276- 

the  temperate  zone  a  diminution  of  i°  C.  corresponds  in  the  mean 
to  an  ascent  of  180  yards. 

The  cooling  on  ascending  in  the  atmosphere  has  been  observed 
in  balloon  ascents,  and  a  proof  of  it  is  seen  in  the  perpetual  snows 
which  cover  the  highest  mountains,  even  under  the  torrid  zones. 
The  height  at  which  snow  remains  unmelted  through  the  year,  or 
the  line  of  perpetual  snow  met  with,  differs  in  different  places.  On 
the  Andes  it  commences  at  a  height  of  14,760  feet,  and  on  the  Alps 
at  8,880  feet. 

Direction  of  winds.  As  winds  share  the  temperature  of  the  coun- 
tries which  they  have  traversed,  their  direction  exercises  great 
influence  on  the  air  in  any  place.  In  our  climate  the  hottest  winds 
are  the  south,  then  come  the  south-east,  the  south-west,  the  west, 
the  east,  the  north-west,  notfh,  and  lastly,  the  north-east,  which  is 
the  coldest.  The  character  of  the  wind  changes  with  the  seasons  ; 
the  east-wind,  which  is  cold  in  winter,  is  hot  in  summer. 

Proximity  of  the  seas.  The  neighbourhood  of  the  sea  tends  to 
render  the  temperature  of  the  air  uniform,  by  heating  it  in  winter, 
and  cooling  it  in  summer.  The  average  temperature  of  the  sea  in 
equatorial  and  polar  countries  is  always  higher  than  that  of  the 
atmosphere.  With  reference  to  the  uniformity  of  the  temperature, 
it  has  been  found  that  in  temperate  regions,  that  is,  from  25°  to  50° 
of  latitude,  the  difference  between  the  maximum  and  minimum 
temperature  of  a  day  does  not  exceed,  on  the  sea,  2°  to  3°  ;  while 
on  land  it  amounts  to  12°  to  15°.  In  islands  the  uniformity  of 
temperature  is  very  perceptible,  even  during  the  greatest  heats.  In 
continents,  on  the  contrary,  the  winters  for  the  same  latitudes 
become  colder,  and  the  difference  between  the  temperature  of 
summer  and  winter  becomes  greater. 

277.  Gulf  stream. — A  similar  influence  to  that  of  the  winds  is 
exerted  by  currents  of  warm  water.  To  one  of  these,  the  Gulf 
stream,  the  mildness  of  the  climate  in  the  north-west  of  Europe  is 
usually  assigned.  This  great  body  of  water,  taking  its  origin  in 
equatorial  regions,  flows  through  the  Gulf  of  Mexico,  from  whence 
it  derives  its  name  ;  passing  by  the  southern  shores  of  North 
America  it  makes  its  way  -in  a  north-westerly  direction  across  the 
Atlantic,  and  finally  washes  the  coast  of  Ireland  and  the  north-west 
of  Europe  generally.  Its  temperature  in  the  Gulf  is  about  28°  C.  ; 
and  generally  is  a  little  more  than  5°  C.  higher  than  the  rest  of 
the  ocean  on  which  it  floats,  owing  to  its  lower  specific  gravity.  To 


-279]  Climate.  277 

its  influence  is  due  the  milder  climate  of  western  Europe,  as  com- 
pared with  that  of  the  opposite  coast  of  America  ;  thus  the  river 
Hudson,  which  is  in  the  same  latitude  as  Rome,  is  frozen  over  three 
months  in  the  year.  It  also  causes  the  polar  regions  to  be  separa- 
ted from  the  coasts  of  Europe  by  a  girdle  of  open  sea  ;  and  hence 
the  harbour  of  Hammerfest  is  open  the  year  round.  Besides  its  in- 
fluence in  thus  moderating  climate,  the  Gulf  stream  is  an  important 
help  to  navigators. 

278.  isothermal  lines. — When  on  a  map  all  the  points  whose 
temperature  is  known  to  be  the  same  are  joined,  curves  are  obtained 
which  Humboldt  first  noticed,  and  which  he  called  isothermal  lines. 
If  the  temperature  of  a  place  only  varied  with  the  obliquity  of 
the  sun's  rays,  that  is,  with  the  latitude,  isothermal  lines  would  all 
be  parallel  to  the   equator ;  but  as  the  temperature  is  influenced 
by  many  local  causes,  especially  by  the  height,  the  isothermal  lines, 
are  always  more  or  less  curved.     On  the  sea,  however,  they  are 
almost  parallel.     A  distinction  is  made  between  isothermal  lines, 
tsotheral  lines,  and  isochimenal  lines,  where  the  mean  general,  the 
mean  summer,  and  the  mean  winter  temperatures  are  respectively 
constant.     An  isothermal  zone  is  the  space  comprised  between  two 
isothermal  lines.      Kupffer  also  distinguishes  isogeothermal  lines, 
where  the  mean  temperature  of  the  soil  is  constant. 

279.  Climate. — By   the  climate  of  a  place  is  understood  the 
whole  of  the  meteorological  conditions  to  which  a  place  is  sub- 
jected ;  its  mean  annual  temperature,  summer  and  winter  tempera- 
tures, and  by  the  extremes  within   which  these   are  comprised. 
Some  writers  distinguish  seven  classes  of  climates,  according  to 
their  mean  annual  temperature,  a  hot  climate  from  30°  to  25°  C. ;  a 
warm  climate  from  25°  to  20°  C. ;  a  mild  climate  from  20°  to  15°  C. ; 
a  temperate  climate  from  15°  to  10°  C. ;  a  cold  climate  from  10°  to 
5°  C.  ;  a  very  cold  climate  from  5°  to  zero  ;  and  an  arctic  climate 
where  the  temperature  is  below  zero. 

Those  climates  again,  are  classed  as  constant  climates,  where 
the  difference  between  the  mean  and  summer  and  winter  tempera- 
ture does  not  exceed  6°  to  8°  ;  variable  climates,  where  the  differ- 
ence amounts  to  from  16°  to  20°  ;  and  extreme  climates,  where  the 
difference  is  greater  than  30°.  The  climates  of  Paris  and  London 
are  variable  ;  those  of  Pekin  and  New  York  are  extreme.  Island 
climates  are  generally  little  variable,  as  the  temperature  of  the  sea 
is  constant  ;  and  hence  the  distinction  between  land  and  sea  cli- 
mates. Marine  climates  are  characterised  by  the  fact,  that  the 


278  On  Heat.  [279- 

difference  between  the  temperature  of  summer  and  winter  is  always 
less  than  in  the  case  of  continental  climates.  But  the  temperature 
is  by  no  means  the  only  character  which  influences  climates  ; 
there  are,  in  addition,  the  moisture  of  the  air,  the  quantity  and 
frequency  of  the  rains,  the  number  of  storms,  the  direction  and  in- 
tensity of  the  winds,  and  the  nature  of  the  soil. 


FOG.      RAIN.      DEW. 

280.  Fog-s  and  mists. — When  aqueous  vapours,  rising  from  a 
vessel  of  boiling  water,  diffuse  in  the  colder  air,  they  are  condensed  ; 
a  sort  of  cloud  is  formed  which  consists  of  a  number  of  small 
hollow  vesicles  of  water,  which  remain  suspended  in  the  air.     These 
are  usually  spoken  of  as  vapours,  yet  they  are  not  so,  at  any  rate 
not  in  the  physical  sense  of  the  word  ;  for  they  are  partially  con- 
densed vapours. 

When  this  condensation  of  aqueous  vapours  is  not  occasioned 
by  contact  with  cold  solid  bodies,  but  takes  place  throughout  large 
spaces  of  the  atmosphere,  the  effect  is  to  form,  fogs  or  mists,  which, 
in  fact,  are  nothing  more  than  the  appearance  seen  over  a  vessel 
of  hot  water. 

A  chief  cause  of  fogs  consists  in  the  moist  soil  being  at  a  higher 
temperature  than  the  air.  The  vapours  which  then  ascend  con- 
dense and  become  visible.  In  all  cases,  however,  the  air  must 
have  reached  its  point  of  saturation  before  condensation  takes 
place.  Fogs  may  also  be  produced  when  a  current  of  hot  and 
moist  air  passes  over  a  river  at  a  lower  temperature  than  its  own, 
for  then  the  air  being  cooled,  as  soon  as  it  is  saturated  the  excess 
of  vapour  present  is  condensed. 

The  distinction  between  mists  and  fogs  is  one  of  degree  rather 
than  of  kind.  A  fog  is  a  very  thick  mist. 

281.  Clouds. — Clouds   are  masses  of  vapour,  cqndensed  into 
little  drops  or  vesicles  of  extreme  minuteness,  like  fogs  ;  from  which 
they  only  differ  in  occupying  the  higher  regions  of  the  atmosphere  ; 
they  always  result  from  the  condensation  of  vapours  which  rise 
from  the  earth  or  the  sea.     According  to  their  appearance,  they 
have  been 'divided  by  Howard  into  four  principal  kinds  :  the  nim- 
bus, the  stratus,  the  cumulus,  and  the  cirrus.     These  four  kinds  are 
represented  in  fig.  221,  and  are  designated  respectively  by  one,  two, 
three,  and  four  birds  on  the  wing. 

The  cirrus  consist  of  small  whitish  clouds,  which  have  a  fibrous 


-281] 


Clouds. 


279 


or  wispy  appearance,  and  occupy  the  highest  regions  of  the  atmo- 
sphere. The  name  of  mare's  tails,  by  which  they  are  generally 
known,  well  describes  their  appearance.  From  the  low  tempera- 
ture of  the  spaces  which  they  occupy,  it  is  more  than  probable  that 
cirrus  clouds  consist  of  frozen  particles  ;  and  hence  it  is  that  haloes, 
coronas,  and  other  optical  appearances,  produced  by  refraction  and 
reflection  from  ice  crystals,  appear  almost  always  in  these  clouds 
and  their  derivatives.  Their  appearance  often  precedes  a  change 
of  weather. 

The  cumulus  are  rounded  spherical  forms  which  look  like  moun- 


Fig.  221. 


tains  piled  one  on  the  other.  They  are  more  frequent  in  summer 
than  in  winter,  and,  after  being  formed  in  the  morning,  they  gene- 
rally disappear  towards  evening.  If,  on  the  contrary,  they  become 
more  numerous,  and  especially  if  surmounted  by  cirrus  clouds,  rain 
or  storms  may  be  expected. 

Stratus  clouds  consist  of  very  large  and  continuous  horizontal 
sheets,   which   chiefly   form  at   sunset,  and  disappear  at  sunrise; 


28o  On  Heat.  [281- 

They  are  frequent  in  autumn  and  unusual  in  spring  time,  and  are 
lower  than  the  proceeding. 

The  nimbus,  or  rain  clouds,  which  are  sometimes  classed  as  one 
of  the  fundamental  varieties,  are  properly  a  combination  of  the 
three  preceding  kinds.  They  affect  no  particular  form,  and  are 
solely  distinguished  by  a  uniform  grey  tint,  and  by  fringed  edges. 
They  are  indicated  on  the  right  of  the  figure  by  the  presence  of  one 
bird. 

The  fundamental  forms  pass  into  one  another  in  the  most  varied 
manner  ;  Howard  has  classed  these  traditional  forms  as  cirro- 
cumulus,  firro-stratus,  and  cumulo-stratus,  and  it  is  often  very 
difficult  to  tell,  from  the  appearance  of  a  cloud,  which  type  it  most 
resembles.  The  cirro-cumulus  is  most  characteristically  known  as 
a  '  mackerel  sky  ; '  it  consists  of  small  roundish  masses,  disposed 
with  more  or  less  irregularity  and  Connection.  It  is  frequent  in 
summer,  and  attendant  on  warm  and  dry  weather.  Cirro-stratus 
appears  to  result  from  the  subsidence  of  the  fibres  of  cirrus  to  a 
horizontal  position,  at  the  same  time  that  they  approach  each  other 
laterally.  The  form  and  relative  position  when  seen  in  the  distance 
frequently  give  the  idea  of  shoals  of  fish.  The  tendency  of  cumulo- 
stratus  is  to  spread,  settle  down  into  the  nimbus,  and  finally  fall  as 
rain. 

The  height  of  clouds  varies  greatly  ;  in  the  mean  it  is  from 
1,300  to  1,500  yards  in  winter,  and  from  3,300  to  4,400  yards  in 
summer.  But  they  often  exist  at  greater  heights  ;  Gay-Lussac,  in 
his  balloon  ascent,  at  a  height  of  7,650  yards,  observed  cirrus- 
clouds  above  him,  which  appeared  still  to  be  at  a  considerable 
height.  In  Ethiopia  M.  d'Abbadie  observed  storm-clouds  whose 
height  was  only  230  yards  above  the  ground. 

In  order  to  explain  the  suspension  of  clouds  in  the  atmosphere, 
H  alley  first  proposed  the  hypothesis  of  vesicular  vapours.  He  sup- 
posed that  clouds  are  formed  of  an  infinity  of  extremely  minute 
vesicles,  hollow,  like  soap  bubbles  filled  with  air,  which  is  hotter 
than  the  surrounding  air  ;  so  that  these  vesicles  float  in  the  air 
like  so  many  small  balloons.  This  theory  has  at  present  many 
opponents,  who  assume  that  clouds  and  fogs  consist  of  extremely 
minute  droplets  of  water,  which  are  retained  in  the  atmosphere  by 
the  ascensional  force  of  currents  of  hot  air,  just  as  light  powders  are 
raised  by  the  wind.  Ordinarily,  clouds  do  not  appear  to  descend, 
but  this  absence  of  downward  motion  is  only  apparent.  In  fact, 
clouds  do  usually  fall  slowly,  but  then  the  lower  part  is  continually 


-283]  Rain.  28  [ 

dissipated  on  coming  in  contact  with  the  lower  and  more  heated 
layers  ;  at  the  same  time  the  upper  part  is  always  increasing  from 
the  condensation  of  new  vapours,  so  that  from  these  two  actions 
clouds  appear  to  retain  the  same  height. 

282.  Formation  of  clouds. — Many  causes  may  concur  in  the 
formation  of  clouds.     I.  The  low  temperature  of  the  higher  regions 
of  the  atmosphere.     For  owing  to  the  solar  radiation,  vapours  are 
constantly  disengaged  from  the  earth  and  from  the  waters,  which 
from  their  elastic  force  and  lower  density  rise  in  the  atmosphere  ; 
meeting  there  continually  colder  and  colder  layers  of  air,  they  sink 
to  the  point  of  saturation,  and  then  condensing  in  infinitely  small 
droplets,  they  give  rise  to  clouds. 

II.  The  hot  and  moist  currents  of  air  rising  during  the  day 
undergo  a  gradually  feebler  pressure,  and  thus  is  produced  an 
expansion,  which  is  a  source  of  intense  cold,  and  produces  a  con- 
densation of  vapour.     Hence  it  is  that  high  mountains,  stopping 
the  aerial  currents,  and  forcing  them  to  rise,  are  an  abundant  source 
of  rain. 

III.  A  hot,  moist  current  of  air  mixing  with  a  colder  current, 
undergoes  a  cooling,  which  brings  about  a  condensation  of  the 
vapour.   Thus  the  hot  and  moist  winds  of*  the  south  and  south-west, 
mixing  with  the  colder  air  of  our  latitudes,  give  rain.    The  winds,  of 
the  north  and  north-east  tend  also,  in  mixing  with  our  atmosphere, 
to  condense  the  vapours ;  but  as  these  winds,  owing  to  their  low 
temperature,  are  very  dry,  the  mixture  rarely  attains  saturation, 
and  generally  gives  no  rain. 

283.  Rain. — When,  by  the  constant  condensation  of  aqueous 
vapour,  the  individual  vapour  vesicles  become  larger  and  heavier, 
and  when  finally  individual  vesicles  unite,  they  form  regular  drops, 
which  fall  as  rain.     The  quantity  of  rain  which  falls  annually  in 
any  given  place,  or  the  annual  rainfall,  is  measured  by  means  of  a 
rain  gaitge  or  pluviometer. 

Many  local  circumstances  may  effect  the  quantity  of  rain  which 
falls  in  different  countries  ;  but,  other  things  being  equal,  most  rain 
falls  in  hot  climates,  for  there  the  vaporisation  is  most  abundant. 
The  rain-fall  decreases,  in  fact  from  the  equator  to  the  poles.  At 
London  it  is  23-5  inches ;  at  Bordeaux  it  is  25*8  ;  at  Madeira  it 
is  277  ;  at  Havannah  it  is  91-2  ;  and  at  St.  Domingo  it  is 
107-6.  The  quantity  varies  with  the  seasons  ;  in  Paris,  in  winter 
it  is  4'2  inches  ;  in  spring  6-9  ;  in  summer  6-3  ;  and  in  autumn 
4-8  inches. 


282  On  Heat.  [283- 

An  inch  of  rain  on  a  square  yard  of  surface  expresses  a  fall  of 
4674  pounds,  or  4-67  gallons.  On  an  acre  it  corresponds  to  22,622 
gallons,  or  100-9935  tons.  100  tons  per  inch  per  acre  is  a  ready 
way  of  remembering  this. 

284.  Dew.  Hoarfrost. — Dew  is  merely  aqueous  vapour  which 
has  condensed  on  bodies  during  the  night  in  the  form  of  minute, 
globules.  It  is  occasioned  by  the  chilling  which  bodies  near  the 
surface  of  the  earth  experience  in  consequence  of  nocturnal  radia- 
tion. Their  temperature  having  then  sank  several  degrees  below 
that  of  the  air,  it  frequently  happens,  especially  in  hot  seasons, 
that  this  temperature  is  below  that  at  which  the  atmosphere  is 
saturated.  The  layer  of  air  which  is  immediately  in  contact  with 
the  chilled  bodies,  and  which  virtually  has  the  same  temperature, 
then  deposits  a  portion  of  the  vapour  which  it  contains  :  just  as 
when  a  bottle  of  cold  water  is  brought  into  a  warm  room,  it  be- 
comes covered  with  moisture,  owing  to  the  condensation  of  aqueous 
vapour  upon  it. 

According  to  this  theory,  which  was  first  propounded  by  Dr. 
Wells,  all  causes  which  promote  the  cooling  of  bodies  increase  the 
quantity  of  dew.  These  causes  are  the  emissive  power  of  bodies, 
the  state  of  the  sky,  and  the  agitation  of  the  air.  Bodies  which 
have  a  great  radiating  power  more  readily  become  cool,  and  there- 
fore ought  to  condense  more  vapour.  In  fact  there  is  generally 
no  deposit  of  dew  on  metals,  whose  radiating  power  is  very  small, 
especially  when  they  are  polished ;  while  the  ground,  sand,  glass, 
and  plants,  which  have  a  great  radiating  power,  become  abundantly 
covered  with  dew.  On  some  plants,  for  instance,  not  merely  are 
droplets  of  dew  formed,  but  regular  layers  of  water. 

The  state  of  the  sky  also  exercises  a  great  influence  on  the  for- 
mation of  dew.  If  the  sky  is  cloudless,  the  planetary  spaces  send 
to  the  earth  an  inappreciable  quantity  of  heat,  while  the  earth 
radiates  very  considerably,  and  therefore  becoming  very  much 
chilled,  there  is  an  abundant  deposit  of  dew.  But  if  there  are 
clouds,  as  their  temperature  is  far  higher  than  that  of  the  planetary 
spaces,  they  radiate  in  turn  towards  the  earth,  and  as  bodies  on  the 
surface  of  the  earth  only  experience  a  feeble  chilling,  no  deposit  of 
dew  takes  place 

Wind  also  influences  the  quantity  of  vapour  deposited.  If  it  is 
feeble,  it  increases  it,  inasmuch  as  it  renews  the  air ;  if  it  is  strong, 
it  diminishes  it,  as  it  heats  the  bodies  by  contact,  and  thus  does  not 
the  air  time  to  become  cooled.  Finally,  the  deposit  of  dew  is 


-285] 


Snow.     Sleet. 


283 


more  abundant  according  as  the  air  is  moister,  for  then  it  is  nearer 
its  point  of  saturation. 

Hoar  frost  and  rime  are  nothing  more  than  dew  which  has 
been  deposited  on  bodies  cooled  below  zero,  and  has  therefore 
become  frozen.  The  flocculent  form  which  the  small  crystals  pre- 
sent, of  which  rime  is  formed,  shows  that  the  vapours  solidify 
directly,  without  passing  through  the  liquid  state.  Hoar  frost, 
like  dew,  is  formed  on  bodies  which  radiate  most,  such  as  the 
stalks  and  leaves  of  vegetables,  and  is  chiefly  deposited  on  the 
parts  turned  towards  the  sky. 

285.  Snow.  Sleet.  Snow  is  water  solidified  in  stellate  crystals, 
variously  modified,  and  floating  in  the  atmosphere.  These  crystals 
arise  from  the  congelation  of  the  minute  vesicles  which  constitute 
the  clouds,  when  the  temperature  of  the  latter  is  below  zero.  They 


Fig.   222. 

are  more  regular  when  formed  in  a  calm  atmosphere.  Their  form 
may  be  investigated  by  collecting  them  on  a  black  surface,  and 
viewing  them  through  a  strong  lens.  The  regularity,  and  at  the 
same  time  variety,  of  their  forms  are  truly  beautiful.  Fig.  222 
shows  some  of  the  forms  as  seen  through  a  microscope. 

It  snows  most  in  countries  near  the  poles,  or  which  are  high 
above  the  sea  level.  Towards  the  poles,  the  earth  is  constantly 
covered  with  snow  ;  the  same  is  the  case  on  high  mountains,  where 
there  are  perpetual  snows  even  in  equatorial  countries. 


284  On  Heat.  [285- 

Sleet  is  also  solidified  water,  and  consists  of  small  icy  needles 
pressed  together  in  a  confused  manner.  Its  formation  is  ascribed 
to  the  sudden  congelation  of  the  minute  globules  of  the  clouds  in 
an  agitated  atmosphere. 

286.  Bail. — Hail  is  a  mass  of  compact. globules  of  ice  of  different 
sizes,  which  fall  in  the  atmosphere.  In  our  climates  hail  falls 
principally  during  spring  and  summer,  and  at  the  hottest  times  of 
the  day  :  it  rarely  falls  at  night.  The  fall  of  hail  is  always  pre- 
ceded by  a  peculiar  noise.  Hail  is  generally  the  precursor  of  storms,, 
it  rarely  accompanies  them,  and  follows  them  more  rarely  still. 
A  hailstone  consists  of  a  core  of  snow,  which  is  surrounded  by  con- 
centric layers  of  ice.  Hail  falls  from  the  size  of  small  peas  to  that 
of  an  egg  or  an  orange.  The  formation  of  hailstones,  and  more 
especially  their  great  size,  have  never  been  altogether  satisfactorily 
accounted  for.  While  snow  sometimes  falls  for  days  together,  hail- 
storms seldom  last  longer  than  a  quarter  of  an  hour,  and  they  are 
also  far  less  frequent. 


ON  WINDS   IN   GENERAL. 

287.  Direction  and  velocity  of  winds. —  Winds  are  currents 
moving  in  the  atmosphere  with  variable  directions  and  velocities. 
There  are  eight  principal  directions  in  which  they  blow  :  north, 
north-east,  east,  south-east,  south,  south-west,  west,  and  north-west. 
Mariners  further  divide  each  of  the  distances  between  these  eight 
directions  into  four  others,  making  in  all  32  directions,  which  are 
called  points  or  rhumbs.  A  figure  of  these  32  rhumbs  on  a  circle 
in  the  form  of  a  star,  is  known  as  the  mariner's  card. 

The  direction  of  the  wind  is  determined  by  means  of  vanes,  and 
its  velocity  by  means  of  the  anemometer.  There  are  several  forms 
of  this  instrument ;  the  most  usual  consists  of  a  small  vane  with 
fans,  which  the  wind  turns  ;  the  velocity  is  deduced  from  the 
number  of  turns  made  in  a  given  time,  which  is  measured  by 
means  of  an  endless  screw  and  wheel-work.  In  our  climate  the 
mean  velocity  is  from  1 8  to  20  feet  in  a  second.  With  a .  velocity 
of  6  or  7  feet,  the  wind  is  moderate ;  with  30  or  35  feet,  it  is  fresh  ; 
with  60  or  70  feet,  it  is  strong  ;  with  a  velocity  of  85  to  90  feet,  it  is 
a  tempest,  and  from  90  to  120  it  is  a  hurricane. 

288.  Causes  of  winds. — Winds  are  produced  by  the  disturbance 
of  the  equilibrium  in  some  part  of  the  atmosphere,  a  disturbance 


-289]  Winds.  285 

always  resulting  from  a  difference  in  temperature  between  adjacent 
countries.  Thus,  if  the  temperature  of  a  certain  extent  of  ground 
becomes  higher,  the  air  in  contact  with  it  becomes  heated,  it  ex- 
pands, and  rises  towards  the  higher  regions  of  the  atmosphere  ; 
whence  it  flows,  producing  winds  which  blow  from  hot  to  cold 
countries.  But  at  the  same  time  the  equilibrium  is  destroyed  at 
the  surface  of  the  earth,  for  the  barometric  pressure  on  the  colder 
adjacent  parts  is  greater  than  on  that  which  has  been  heated,  and 
hence  a  current  will  be  produced  with  a  velocity  dependent  on  the 
difference  between  these  pressures  ;  thus  two  distinct  winds  will 
be  produced,  an  upper  one  setting  outwards  from  the  heated  region, 
and  a  lower  one  setting  inwards  towards  it. 

289.  Regular,  periodical,  and  variable  winds. — According  to 
the  more  or  less  constant  directions  in  which  winds  blow,  they  may 
be  classed  as  regular,  periodical,  and  variable  winds. 

i.  Regular  winds  are  those  which  blow  all  the  year  through  in 
a  virtually  constant  direction.  These  winds,  which  are  also  known 
as  the  trade  winds,  are  uninterruptedly  observed  far  from  the  land 
in  equatorial  regions,  blowing  from  the  north-east  to  the  south-west 
in  the  northern  hemisphere,  and  from  the  south-east  to  the  north- 
west in  the  southern  hemisphere.  They  prevail  on  the  two  sides 
of  the  equator  as  far  as  30°  of  latitude,  and  they  blow  m  the 
same  direction  as  the  apparent  motion  of  the  sun,  that  is,  from 
east  to  west. 

The  air  above  the  equator  being  gradually  heated,  rises  as  the 
sun  passes  round  from  east  to  west,  and  its  place  is  supplied  by 
the  colder  air  from  the  north  or  south.  The  direction  of  the  wind, 
however,  is  modified  by  this  fact  ;  that  the  velocity  which  this 
colder  air  has  derived  from  the  rotation  of  the  earth,  namely,  the 
velocity  of  the  surface  of  the  earth  at  that  point  from  which  it 
started,  is  less  than  the  velocity  of  the  surface  of  the  earth  at  the 
point  at  which  it  has  now  arrived ;  hence  the  currents  acquire,  in 
reference  to  the  equator,  the  constant  direction  which  constitutes 
the  trade  winds. 

ii.  Periodical  winds  are  those  which  blow  regularly  in  the  same 
direction  at  the  same  seasons,  and  at  the  same  hours  of  the  day  ; 
the  monsoon,  simoom,  and  the  land  and  sea  breeze  are  examples 
of  this  class.  The  name  monsoon  is  given  to  winds  which  blow 
for  six  months  in  one  direction,  and  for  six  months  in  another. 
They  are  principally  observed  in  the  Red  Sea  and  in  the  Arabian 
Gulf,  in  the  Bay  of  Bengal  and  in  the  Chinese  Sea.  These  winds 


286  On  Heat.  [289- 

blow  towards  the  continents  in  summer,  and  in  a  contrary  direc- 
tion in  winter.  The  simoom  is  a  hot  wind  which  blows  over  the 
deserts  of  Asia  and  Africa,  and  which  is  characterised  by  its  high 
temperature  and  by  the  sands  which  it  raises  in  the  atmosphere 
and  carries  with  it.  During  the  prevalence  of  this  wind  the  air  is 
darkened,  the  skin  feels  dry,  the  respiration  is  accelerated,  and  a 
burning  thirst  is  experienced. 

This  wind  is  known  under  the  name  of  sirocco  in  Italy  and 
Algiers,  where  it  blows  from  the  great  desert  of  Sahara.  During 
its  prevalence  people  remain  at  home,  the  windows  and  doors 
being  carefully  closed.  In  Egypt,  where  it  prevails  from  the  end 
of  April  to  June,  it  is  called  kamsin,  from  a  word  signifying  fifty  ; 
for  it  lasts  ordinarily  50  days  ;  25  before  the  spring  equinox,  and 
25  after.  When  caravans  are  surprised  by  this  wind,  men  cover 
their  faces  with  thick  clothes,  and  camels  turn  their  backs  to  the 
torment.  The  natives  of  Africa,  in  order  to  protect  themselves 
from  the  effects  of  the  too  rapid  perspiration  occasioned  by  this 
wind,  cover  themselves  with  fatty  substances. 

The  land  and  sea  breeze  is  a  wind  which  blows  on  the  sea  coast 
during  the  day  from  the  sea  towards  the  land,  and  during  the  night 
from  the  land  to  the  sea.  For  during  the  day  the  land  becomes 
more  heated  than  the  sea,  in  consequence  of  its  lower  specific 
heat  (257)  and  greater  conductivity,  and  hence  as  the  superincum- 
bent air  becomes  more  heated  than  that  upon  the  sea,  it  ascends 
and  is  replaced  by  a  current  of  colder  and  denser  air  flowing  from 
the  sea  towards  the  land.  During  the  night  the  land  cools  more 
rapidly  than  the  sea,  and  hence  the  same  phenomenon  is  produced 
in  a  contrary  direction.  The  sea  breeze  commences  after  sunrise, 
increases  to  three  o'clock  in  the  afternoon,  decreases  towards 
evening,  and  is  changed  into  the  land  breeze  after  sunset.  These 
winds  are  only  perceived  at  a  slight  distance  from  the  shores. 
They  are  regular  in  the  tropics,  but  less  so  in  our  climates  ;  and 
traces  of  them  are  seen  as  far  as  the  coasts  of  Greenland.  The 
proximity  of  mountains  also  gives  rise  to  periodical  daily  breezes. 

iii.  Variable  winds  are  those  which  blow  sometimes  in  one 
direction  and  sometimes  .in  another,  alternately,  without  being 
subject  to  any  law.  In  mean  latitudes  the  direction  of  the  winds 
is  very  variable  ;  towards  the  poles  this  irregularity  increases,  and 
under  the  arctic  zone  the  winds  frequently  blow  from  several  points 
of  the  horizon  at  once.  On  the  other  hand,  in  approaching  the 
torrid  zone,  they  become  more  regular.  The  south-west  wind 


-292]  Sources  of  Heat  and  Cold.  287 

prevails  in  the  north  of  France,  in  England,  and  in  Germany;  in 
the  south  of  France  the  direction  inclines  towards  the  north,  and 
in  Spain  and  Italy  the  north  wind  predominates. 

290.  law  of  the  rotation  of  winds. — Spite  of  the  great  irregu- 
larity which  characterises  the  direction  of  the  winds  in  our  latitude, 
it  has  been  ascertained  that  the  wind  has  a  preponderating  ten- 
dency to  veer  round  according  to  the  sun's  motion  ;  that  is,  to  pass 
from  north,  through  north-east,  east,  south-east  to  south,  and  so  on 
round  in  the  same  direction  from  west  to  north  ;  that  it  often  makes 
a  complete  circuit  in  that  direction,  or  more  than  one  in  succes- 
sion, occupying  many  days  in  doing  so,  but  that  it  rarely  veers,  and 
very  rarely  or  never  makes  a  complete  circuit  in  the  opposite  direc- 
tion. 

For  a  station  in  south  latitude  a  contrary  law  of  rotation  pre- 
vails. 

This  law,  though  more  or  less  suspected  for  a  long  time,  was 
first  formally  enunciated  and  explained  by  Dove,  and  is  known  as 
Dove's  law  of  rotation  of  winds. 


CHAPTER  XIII. 

SOURCES  OF   HEAT  AND   COLD. 

291.  Different  sources  of  heat. — The  following  different  sources 
of  heat  may  be  distinguished  :  i.  the  mechanical  sources,  comprising 
friction,  percussion,  and  pressure  ;  ii.  the  physical  sources — that  is, 
solar  radiation,  terrestrial  heat,  the  molecular  actions,  the  changes 
of  condition  and  electricity  ;  iii.  the  chemical  sources,  or  molecular 
combinations,  and  more  especially  combustion. 

MECHANICAL  SOURCES. 

292.  Beat  due  to  friction. — The  friction  of  two  bodies,  one 
against  the  other,  produces  heat,  which  is  greater  the  greater  the 
pressure  and  the  more  rapid  the  motion.     For  example,  the  axles 
of  carriage  wheels,  by  their  friction  against  the  boxes,  often  become 
so  strongly  heated  as  to  take  fire.     By  rubbing  together  two  pieces 
of  ice  in  a  vacuum  below  zero,  Sir  H.  Davy  partially  melted  them. 


288  On  Heat.  [292- 

In  boring  a  brass  cannon  Rumford  found  that  the  heat  developed 
in  the  course  of  2^  hours  was  sufficient  to  raise  26^  pounds  of  water 
from  zero  to  the  boiling  point. 

This  may  be  well  illustrated  by  an  experiment,  fig.  223,  devised 
by  Prof.  Tyndal.  A  brass  tube,  b,  closed  at  the  bottom,  about 
4  inches  long  and  less  than  an  inch  in  diameter,  fits  on  the  whirling 
table,  having  been  three-quarters  filled  with  cold  water  and  corked. 


Fig.  223. 

If  now  it  be  clasped  by  a  sort  of  wooden  squeezer  in  which  there 
are  two  semicircular  grooves,  and  then  be  made  to  rotate,  the  heat 
developed  by  the  friction  is  sufficient  to  boil  the  water  and  expel 
the  cork  by  which  it  is  closed. 

293.  Heat  due  to  pressure  and  percussion. —  If  a  body  be  SO 
compressed  that  its  density  is  increased,  its  temperature  rises  ac- 
cording as  the  volume  diminishes.  In  solids  and  liquids,  which  are 
but  little  compressible,  ihe  disengagement  of  heat  is  not  great ; 
though  Joule  has  verified  it  in  the  case  of  water  and  of  oil,  which 
were  exposed  to  pressures  of  15  to  25  atmospheres.  Similarly, 
when  weights  are  laid  on  metallic  pillars,  heat  is  evolved,  and  ab- 
sorbed when  they  are  removed. 

The  production  of  heat  by  the  compression  of  gases  is  easily 
shown  by  means  of  the  pneumatic  syringe,  (fig.  224).  This  consists 
of  a  glass  tube  with  thick  sides,  closed  hermetically  by  a  leathern 
piston.  At  the  bottom  of  this,  there  is  a  cavity  in  which  a  small 
piece  of  tinder  is  placed.  The  tube  being  full  of  air  the  piston  is 
suddenly  plunged  downwards,  the  air  thus  compressed  disengages 
as  much  heat  as  to  ignite  the  tinder,  which  is  seen  to  burn  when  the 
piston  is  rapidly  withdrawn.  The  inflammation  of  the  tinder  in 
this  experiment  indicates  a  temperature  of  at  least  300°.  At  the 
moment  of  compression  a  bright  flash  is  observed,  which  was 


-294] 


PJiysical  Sources  of  Heat. 


289 


originally  attributed  to  the  high  temperature  of  the  air  ;  but  it  is 
simply  due  to  the  combustion  of  the  oil  which  greases  the  piston. 


Fig.  224. 

Percussion  is  also  a  source  of  heat,  as  is  observed  in  the  sparks 
which  are  thrown  off  by  horses  in  trotting  over  a  hard  pavement. 
In  firing  a  shot  at  an  iron  target,  a  sheet  of  flame  is  frequently  seen 
at  the  moment  of  impact ;  and  Mr.  Whitworth  has  used  iron  shells 
which  are  exploded  by  the  concussion  on  striking  an  iron  target. 
A  small  piece  of  iron  hammered  on  the  anvil  becomes  very  hot. 
The  heat  is  not  simply  due  to  an  approximation  of  the  molecules, 
that  is,  to  an  increase  in  density,  but  arises  from  a  vibratory  motion 
imparted  to  them  ;  for  lead,  which  does  not  become  denser  by  being 
hammered,  nevertheless  becomes  heated. 


PHYSICAL  SOURCES. 

294.  Solar  radiation. — The  most  intense  of  all  sources  of  heat 
is  the  sun.  The  cause  of  its  heat  is  unknown  ;  some  have  con- 
sidered it  to  be  an  ignited  mass  experiencing  immense  eruptions, 
while  others  have  regarded  it  as  composed  of  layers  acting 
chemically  on  each  other  like  the  couples  of  a  voltaic  battery,  and 
giving  rise  to  electrical  currents,  which  produce  light  and  solar 
heat.  On  both  hypotheses  the  incandescence  of  the  sun  would  have 
a  limit. 

Different  attempts  have  been  made  to  determine  the  quantity  of 
heat  annually  emitted  by  the  sun.  M.  Pouillet,  by  means  of  an 
apparatus,  which  he  calls  a  pyrheliometer,  has  calculated  that  if 
the  total  quantity  of  heat  which  the  earth  receives  from  the  sun  in 
the  course  of  a  year  were  employed  to  melt  rce,  it  would  be  capable 
of  melting  a  layer  of  ice  all  round  the  earth  of  35  yards  in  thickness. 

U 


290  On  Heat.  [294- 

But  from  the  surface  which  the  air  exposes  to  the  solar  radiation, 
and  from  the  distance  which  separates  the  earth  from  the  sun,  the 
quantity  of  heat  which  the  earth  receives  can  only  be  o^ir.o'oo.ooo  °f 
the  heat  emitted  by  the  sun. 

Faraday  has  calculated  that  the  average  amount  of  heat  radiated 
in  a  day  on  each  acre  of  ground  in  the  latitude  of  London  is  equal 
to  that  which  would  be  produced  by  the  combustion  of  sixty  sacks 
of  coal. 

295.  Terrestrial  beat. — Our  globe  possesses  a  heat  peculiar  to 
it,  which  is  called  the  terrestrial  heat.     The  temperature  of  the 
earth  gradually  sinks  from  the  surface  to   a  certain  depth,  at  which 
it  remains  constant  in  all  seasons.     It  is  hence  concluded  that  the 
sun's  heat  does  not  penetrate  below  a  certain  internal  layer,  which 
is  called  the  layer  of  constant  temperature  :  its  depth  below  the 
earth's  external  surface  varies,  of  cource,  in  different  parts  of  the 
globe  ;  at  Paris  it  is  about  thirty  yards,  and  the  temperature  is  con- 
stant at  i i -8°  C. 

Below  the  layer  of  constant  temperature,  the  temperature  is  ob- 
served to  increase,  on  the  average  i°  C.  for  every  90  feet.  This  in- 
crease has  been  verified  in  mines  and  artesian  wells.  According 
to  this,  at  a  depth  of  3,000  yards,  the  temperature  of  a  corresponding 
layer  would  be  100°,  and  at  a  depth  of  20  to  30  miles  there  would 
be  a  temperature  sufficient  to  melt  all  substances  which  exist  on  the 
surface.  Hot  springs  and  volcanoes  confirm  the  existence  of  this 
central  heat. 

The  heat  produced  by  the  changes  of  condition  has  been  already 
treated  of  in  the  articles  solidification  and  liqtiefaclion  ;  the  Heat 
produced  by  electrical  action  will  be  discussed  under  the  head  of 
ELECTRICITY. 

CHEMICAL  SOURCES. 

296.  Chemical  combinations.     Combustion. — Whenever  two 
bodies  unite  in  virtue  of  their  reciprocal  affinity  this  operation  is 
known  as  the  act  of  chemical  combination.     Chemical  combinations 
are  usually  accompanied  by  a  certain  elevation  of  temperature. 
When  these  combinations  take  place  slowly,  as  when  iron  oxidises 
in  the  air,  and  produces  rust,  the  heat  produced  is  imperceptible  ; 
but  if  they  take  place  rapidly,  the  disengagement  of  heat  is  very 
intense.     The  same  quantity  of  heat  is  produced  in  both  cases,  but 
when  evolved  slowly  it  is  dissipated  as  fast  as  formed. 


-297] 


RumforcTs  Calorimeter. 


291 


Combustion  is  chemical  combination  attended  with  the  evolution, 
of  light  and  heat.  In  the  ordinary  combustion  in  lamps,  fires, 
candles,  the  carbon  and  hydrogen  of  the  coal  or  of  the  oil,  etc., 
combine  with  the  oxygen  of  the  air,  giving  rise  to  aqueous  vapour, 
gases,  and  other  volatile  products  which  are  given  off  as  smoke. 
The  old  expression  that  fire  destroys  everything  is  incorrect.  It 
destroys  nothing,  it  simply  puts  certain  elements  at  liberty  to  unite 
with  others  ;  it  decomposes  but  at  the  same  time  produces.  A  body 
in  being  burned  is  transformed,  but  its  substance  is  not  destroyed. 

Many  combustibles  burn  with  flame.  A  flame  is  a  gas  or  vapour 
raised  to  a  high  temperature  by  combustion.  Its  illuminating 
power  varies  with  the  nature  of  the  products  formed.  The  presence 
of  a  solid  body  in  the  flame  increases  the  illuminating  power.  The 
flames  of  hydrogen,  carbonic  oxide,  and  alcohol  are  pale,  because 
they  only  contain  gaseous  products  of  combustion.  But  the  flames 
of  candles,  lamps,  coal  gas,  have  a  high  illuminating  power.  They 
owe  this  to  the  fact  that  the  high  temperature  produced  decomposes 
certain  of  the  gases  with  the  production  of  carbon,  which,  not  being 
perfectly  burned,  becomes  incandescent  in  the  flame.  Coal  gas, 
when  burnt  in  an  arrangement  by  which  it  obtains  an  adequate 
supply  of  air,  is  almost  entirely  devoid  of  luminosity.  A  non-lumi- 
nous flame  may  be  made 
luminous  by  placing  in  it 
platinum  wire,  or  asbestos. 
The  temperature  of  a 
flame  does  not  depend 
on  its  illuminating  power. 
A  hydrogen  flame,  which 
is  the  palest  of  all  flames^ 
gives  the  greatest  heat. 

297.  i.  umford's  ca- 
lorimeter.—  In  order  to 
determine  the  amount  of 
heat  which  is  produced 
by  combustion,  Rumford 
used  the  calorimeter  de- 
picted in  fig.  225.  A 

metal     box      contains    a  pjg<  225 

known  weight  of  water  at 

a  known  temperature  ;    through  it  passes  a  copper  worm  tube,  ss, 
which  is  open  at  one  end  s',  and  at  the  other  ends  in  a  funnel  c.    The 

U  2 


292  On  Heat.  [297- 

substance  whose  heating  effect  is  to  be  determined,  is  placed  under- 
neath the  funnel,  and  having  been  previously  weighed,  is  ignited. 
The  gaseous  products  of  combustion  pass  then  through  the  worm, 
and  imparting  their  heat  to  the  water,  raise  the  temperature. 
From  the  weight  of  the  water  and  its  increase  in  temperature, 
which  is  measured  by  the  thermometer,  and  from  the  weight  of  the 
body  burned,  its  heating  effect  may  be  determined. 

By  experiments  with  more  perfect  arrangements,  based  however, 
on  the  same  principle,  the  heating  effect  of  the  following  substances 
has  been  determined.  The  numbers  represent  the  number  of 
pounds  of  water  which  are  raised  i°  C.  by  the  combustion  of  a  pound 
of  the  substance. 

Hydrogen  ....  34000          Dry  turf 4800 

Petroleum  ....  12300         Wood 2900 

Coal 6500  Carbonic  oxide  .     .     .  2400 

Phosphorus     .     .     .  5700          Sulphur 2200 


SOURCES   OF   COLD. 

298.  Various  sources  of  cold. — Besides  the  cold  caused  by  the 
passage  of  a  body  from  the  solid  to  the  liquid   state,  of  which  we 
have  already  spoken,  cold  is  produced  by  the  expansion  of  gases,  by 
radiation  in  general,  and  more  especially  by  nocturnal  radiation. 

299.  Cold   produced  toy  the  expansion  of  gases. — We  have 
seen,  that  when  a  gas  is  compressed,  its  temperature  rises.    The 
reverse  of  this  is  also  the  case  :  when  a  gas  is  rarefied  a  reduction 
of  temperature  ensues,  because  a  quantity  of  sensible  heat  disappears 
when  the  gas  becomes  increased  to  a  larger  volume.     This  may  be 
shown  by  placing  a  delicate  Breguet's  thermometer  under  the  re- 
ceiver of  an  air-pump,  and  exhausting ;  at  each  stroke  of  the  piston 
the  needle  moves  in  the  direction  of  zero,  and  regains  its  original 
temperature  when  air  is  admitted.     Kirk  has  invented  a  machine 
for  the  manufacture  of  ice,  which  depends  on  this  property.     The 
heat  developed  by  the  compression  of  air  is  removed  by  a  current 
of  cold  water  ;   the  vessel  containing  the   compressed  air  being 
placed  in  brine,  the  air  is  allowed  to   expand  ;  in  so  doing  it  cools 
the  brine  so  considerably  as  to  freeze  water  contained  in  vessels 
placed  in  the  brine.      It  is  stated  that  by  this  means  a  ton  of  coals 
(used  in  working  a  steam  engine  by  which  the  compression  is  ef- 
fected) can  produce  a  ton  of  ice. 


-300]  Cold  Produced  by  Radiation.  293 

300.  Cold  produced  by  nocturnal  radiation. — During  the  day, 
the  ground  receives  from  the  sun  more  heat  than  radiates  into  space, 
and  the  temperature  rises.  The  reverse  is  the  case  during  night. 
The  heat  which  the  earth  loses  by  radiation  is  no  longer  compen- 
sated for,  and  consequently  a  fall  of  temperature  takes  place,  which 
is  greater  according  as  the  sky  is  clearer,  for  clouds  send  towards 
the  earth  rays  of  greater  intensity  than  those  which  come  from  the 
celestial  spaces.  In  some  winters  it  has  been  found  that  rivers 
have  not  frozen,  the  sky  having  been  cloudy,  although  the  thermo- 
meter has  been  for  several  days  below  -  4° ;  while  in  other  less 
severe  winters  the  rivers  freeze  when  the  sky  is  clear.  The  emissive 
power  exercises  a  great  influence  on  the  cold  produced  by  radiation ; 
the  greater  it  is  the  greater  is  the  cold. 

In  Bengal,  the  nocturnal  cooling  is  used  in  manufacturing  ice. 
Large  flat  vessels  containing  water  are  placed  on  non-conducting 
substances,  such  as  straw  or  dry  leaves.  In  consequence  of  the 
radiation  th,e  water  freezes,  even  when  the  temperature  of  the  air  is 
10°  Cc  The  same  method  can  be  applied  in  all  cases  with  a  clear 
sky. 

It  is  said  that  the  Peruvians  in  order  to  preserve  the  shoots  of 
young  plants  from  freezing,  light  great  fires  in  their  neighbourhood, 
the  smoke  of  which  producing  an  artificial  cloud,  hinders  the 
cooling  produced  by  radiation. 

Country  people  are  in  the  habit  of  saying  that  it  freezes  more 
when  the  moon  appears  than  when  it  is  hidden  by  clouds.  They 
are  right  in  this  ;  but  the  freezing  is  not,  as  they  think,  due  to  the 
influence  of  the  moon.  It  is  owing  to  the  absence  of  .clouds. 


294  'On  Light.  [301 


BOOK    IV. 

ON   LIGHT. 


CHAPTER    I. 
TRANSMISSION,  VELOCITY,   AND    INTENSITY  OF  LIGHT. 

301.  Theories  of  light. — Light  is  the  agent  which,  by  its  action 
on  the  retina,  excites  in  us  the  sensation  of  vision.  That  part  of  phy- 
sics which  deals  with  the  properties  of  light  is  known  as  optics, 

In  order  to  explain  the  origin  of  light,  various  hypotheses  have 
been  made,  the  most  important  of  which  are  the  emission  or  cor- 
puscular theory,  and  the  nndulatory  theory. 

On  the  emission  theory  it  is  assumed  that  luminous  bodies  emit, 
in  all  directions,  an  imponderable  substance,  which .  consists  of 
molecules  of  an  extreme  degree  of  tenuity  :  these  are  propagated 
in  right  lines  with  an  almost  infinite  velocity.  Penetrating  into  the 
eye  they  act  on  the  retina,  and  determine  the  sensation  which  con- 
stitutes vision. 

On  the  undulatory  theory,  all  bodies,  as  well  as  the  celestial 
spaces,  are  filled  by  an  extremely  subtle  elastic  medium,  which  is 
called  the  luminiferous  ether.  The  luminosity  of  a  body  is  due  to 
an  infinitely  rapid  vibratory  motion  of  its  molecules,  which,  when 
communicated  to  the  ether,  is  propagated  in  all  directions  in  the 
form  of  spherical  waves  ;  and  this  vibratory  motion,  being  thus 
transmitted  to  the  retina,  calls  forth  the  sensation  of  vision.  The 
vibrations  of  the  ether  take  place  not  in  the  direction  of  the  wave, 
bu-t  in  a  plane  at  right  angles  to  it.  The  latter  are  called  the  trans- 
versal vibrations.  An  idea  of  these  may  be  formed  by  shaking  a 
rope  at  one  end.  The  vibrations,  or  to  and  fro  movements,  of  the 
particles  of  the  rope,  are  at  right  angles  to  the  length  of  the  rope, 
but  the  onward  motion  of  the  wave's  form  is  in  the  direction  of  the 
length  of  the  rope. 


-302]  Various  Sources  of  L  igJit.  295 

On  the  emission  theory  the  propagation  of  light  is  effected  by 
a  motion  of  translation  of  particles  of  light  thrown  out  from  the 
luminous  body,  as  a  bullet  is  discharged  from  a  gun.  On  the  un- 
dulatory  theory  there  is  no  progressive  motion  of  the  particles 
themselves,  but  only  of  the  state  of  disturbance  which  was  com- 
municated by  the  luminous  body  ;  it  is  a  motion  of  oscillation,  and, 
like  the  propagation  of  waves  in  water,  takes  place  by  a  series  of 
vibrations. 

The  luminiferous  ether  penetrates  all  bodies,  but,  on  account  of 
its  extreme  tenuity,  it  is  uninfluenced  by  gravitation  ;  it  occupies 
space,  and  although  it  presents  no  appreciable  resistance  to  the 
motion  of  the  denser  bodies,  it  is  possible  that  it  hinders  the  motion 
of  the  smaller  comets.  It  has  been  found,  for  example,  that 
Encke's  comet,  whose  period  of  revolution  is  about  3^  years,  has 
its  period  diminished  by  about  0*1 1  of  a  day  at  each  successive 
rotation,  and  this  diminution  is  ascribed  by  some  to  the  resistance 
of  the  ether. 

The  fundamental  principles  of  the  undulatory  theory  were  enun- 
ciated by  Huyghens,  and  subsequently  by  Euler.  The  emission 
theory,  principally  owing  to  Newton's  powerful  support,  was  for 
long  the  prevalent  scientific  creed.  The  undulatory  theory  was 
adopted  and  advocated  by  Young,  who  showed  how  a  large  number 
of  optical  phenomena,  particularly  those  of  diffraction,  were  to  be 
explained  by  that  theory.  Subsequently  to,  though  independently 
of,  Young,  Fresnel  showed  that  the  phenomena  of  diffraction,  and 
also  those  of  polarisation,  are  explicable  on  the  same  theory,  which, 
since  his  time,  has  been  generally  accepted. 

The  undulatory  theory  not  only  explains  the  phenomena  of  light, 
but  it  reveals  an  intimate  connection  between  these  phenomena 
and  those  of  heat ;  it  shows,  also,  how  completely  analogous  the 
phenomena  of  light  are  to  those  of  sound,  regard  being  had  to  the 
differences  of  the  media  in  which  these  two  classes  of  phenomena 
take  place. 

302.  Various  sources  of  light. — The  various  sources  of  light 
are  the  sun,  the  stars,  heat,  chemical  combination,  phosphorescence, 
electricity,  and  meteoric  phenomena. 

The  origin  of  the  light  emitted  by  the  sun  and  by  the  stars  is 
unknown  ;  it  is  assumed  by  some  that  the  ignited  envelope  by  which 
the  sun  is  surrounded  is  gaseous,  and  at  a  very  high  temperature. 

As  regards  the  light  developed  by  heat,  Pouillet  has  observed 
that  bodies  begin  to  be  luminous  in  the  dark  at  a  temperature  of 


296  On  Light.  [302- 

500°  to  600°  ;  above  that  the  light  is  brighter  in  proportion  as  the 
temperature  is  higher. 

The  luminous  effects  witnessed  in  many  chemical  combinations 
are  due  to  the  high  temperatures  produced.  This  is  the  case  with 
the  artificial  lights  used  for  illuminations  ;  for  luminous  flames  are 
nothing  more  than  gaseous  matters  containing  solids  heated  to  the 
point  of  incandescence. 

Phosphorescence  is  the  property  which  a  large  number  of  sub- 
stances possess  of  emitting  light  when  placed  under  certain  condi- 
tions. 

Spontaneous  phosphorescence  is  observed  in  certain  vegetables 
and  animals  ;  for  instance,  it  is  very  intense  in  the  glowworm  and 
in  the  lampyre,  and  the  brightness  of  their  light  appears  to  depend 
on  their  will.  In  tropical  climates  the  sea  is  often  covered  with  a 
bright  phosphorescent  light  due  to  some  extremely  small  zoophytes. 
These  animalculas  emit  a  luminous  matter  so  subtle  that  MM.  Quoy 
and  Gaimard,  during  a  voyage  under  the  equator,  having  placed 
two  in  a  tumbler  of  water,  the  liquid  immediately  became  luminous 
throughout  its  entire  mass. 

Decaying  wood,  and  certain  kinds  of  fish  in  a  state  of  putrefac- 
tion, also  exhibit  this  phenomenon.  Certain  substances,  too,  become 
phosphorescent  by  friction  ;  while  others  become  luminous  in  the 
dark  by  having  been  previously  exposed  to  the  sun's  rays. 

303.  Opaque,  transparent,  translucent  bodies.  Absorption 
of  light. — Bodies  illuminated  by  a  source  of  light  present  two  dis- 
tinct effects  ;  one  class,  such  as  wood,  metals,  most  stones,  com- 
pletely stop  it  ;  while  others,  such  as  air  and  glass,  allow  light  to  pass. 
The  first  class  of  bodies  comprehends  those  which  are  called  opaqiie, 
and  the  second  the  transparent  and  translucent  bodies.  The  term 
transparent  or  diaphanous  is  applied  to  all  bodies  which  at  all 
transmit  light ;  while  transhicency  is  usually  restricted  to  the  case 
of  bodies  through  which  objects  cannot  be  distinctly  seen.  Polished 
glass  may  be  called  either  transparent  or  diaphanous  ;  but  ground 
glass,  oiled  paper,  thin  porcelain,  are  translucent  ;  for,  while  they 
transmit  light,  objects  cannot  be  distinguished  through  them. 

Of  all  bodies  which  transmit  light,  none  can  be  said  to  be  per- 
fectly diaphanous  ;  all  extinguish,  or  absorb,  a  portion  of  the  light 
which  impinges  on  them.  The  most  transparent,  such  as  air,  water, 
glass,  gradually  extinguish  the  light  which  penetrates  them  ;  and  if 
their  thickness  be  considerable,  they  may  weaken  it  so  much  that 
no  impression  is  produced  on  the  eye.  Thus,  on  the  tops  of  high 


-304]  Propagation  of  Light.  297 

mountains  the  number  of  stars  visible  to  the  naked  eye  is  greater 
than  in  the  plain  ;  a  phenomenon  arising  from  the  fact,  that  in  the 
former  case  the  layer  of  air  traversed  is  not  so  thick  as  in  the  latter 
.  case.  In  like  manner  too  the  sun  appears  less  luminous  when  on 
the  horizon,  for  then  its  rays  traverse  thicker  layers  of  air. 

Just  as  there  are  no  perfectly  transparent  substances,  so  too  there 
are  none  which  are  quite  opaque  ;  at  any  rate,  when  the  thickness  is 
inconsiderable.  Gold,  which  is  one  of  the  densest  metals,  when 
beaten  out  in  the 'form  of  fine  leaf,  allows  an  appreciable  quantity  of 
light  to  traverse  it. 

Foucault  has  recently  shown,  that  when  the  object  glass  of  a 
telescope  is  thinly  silvered,  the  layer  is  so  transparent,  that  the  sun 
can  be  viewed  through  it  without  danger  to  the  eyes,  since  the  me- 
tallic layer  reflects  the  greater  part  of  the  heat  and  light  ;  the  tint 
appears  slightly  bluish,  while  in  the  case  of  gold  it  is  greenish. 

304.  Propagation  of  light. — A  medium  is  any  space  or  sub- 
stance which  light  can  traverse,  such  as  a  vacuum,  air,  water,  glass, 
etc.  A  medium  is  said  to  be  homogeneous  when  its  chemical  com- 
position and  density  are  the  same  in  all  parts  ;  conditions  which  are 
independent  of  each  other.  The  atmosphere,  for  instance,  has  every- 
where the  same  composition,  but  not  the  same  density,  owing  to 
the  variations  in  pressure  and  temperature,  to  which  it  is  subject  in 
various  places. 

Experiment  shows  that  in  every  homogeneous  medium  light  is 
propagated  in  a  right  line.  For,  if  an  opaque  body  is  placed  in  the 
right  line  which  joins  the  eye  and  the  luminous  body,  the  light  is 
intercepted.  In  like  manner  we  cannot  receive  any  impression  of 
light  through  a  series  of  holes  in  opaque  plates,  superposed  in  each 
other,  excepting  these  holes  are  in  a  straight  line.  The  light  which 
passes  into  a  dark  room  by  a  small  aperture,  leaves  a  luminous  trace, 
which  is  visible  from  the  light  falling  on  the  particles  suspended  in 
the  atmosphere. 

Light  emanates  from  luminous  bodies  in  all  directions,  for  we  see 
them  equally  in  all  positions  in  which  we  are  placed  round  them. 

Light  changes  its  direction  on  meeting  an  object  which  it  cannot 
penetrate,  or  when  it  passes  from  one  medium  to  another.  These 
phenomena  will  be  described  under  the  heads  reflection  and  re- 
fraction. 

This  emanation  of  light  in  all  directions  about  a  luminous  body 
is  called  radiation,  as  in  the  case  of  heat  ;  a  luminous  ray,  or  ray  of 
light  *  is  the  line  in  which  light  is  propagated  ;  a  luminous  pencil,  or 


298  On  Light.  [304- 

pencil  of  light,  is  a  collection  of  rays  from  the  same  source.  It  is 
said  to  be  parallel,  when  it  is  composed  of  parallel  rays  ;  divergent, 
when  the  rays  separate  from  each  other  ;  and  convergent,  when  they 
tend  towards  the  same  point.  Examples  of  these  will  occur  in  the 
study  of  mirrors  and  of  lenses. 

305.  Shadow.  Penumbra. — When  light  falls  upon  an  opaque 
body,  it  cannot  penetrate  into  the  space  immediately  behind  it,  and 
this  space  is  called  the  shadow. 

In  determining  the  extent  and  the  shape  of  shadow  projected  by 
a  body,  two  cases  are  to  be  distinguished  :  that  in  which  the  lumi- 
nous source  is  a  single  point,  and  that  in  which  it  is  a  body  of  any 
appreciable  extent. 


Fig.  226. 

In  the  first  case,  let  L  (fig.  226)  be  the  luminous  point,.and  M  a 
spherical  body,  which  causes  the  shadow.  If  an  infinitely  long 
straight  line  move  round  the  sphere  M,  always  passing  through  the 
point  L,  this  line  will  produce  a  conical  surface,  which  beyond  the 
sphere,  separates  that  portion  of  space  which  is  in  shadow  from  that 
which  is  illuminated.  In  the  present  case,  on  placing  behind  the 
opaque  body  a  screen,  the  limit  of  the  shadow  will  be  sharply  de- 
fined. This  is  not,  however,  usually  the  case,  for  luminous  bodies 
have  always  a  certain  magnitude,  and  are  not  merely  him: nous 
points  ;  the  shadow  formed  by  a  luminous  point  is  called  the  geo- 
metrical shadow.. 

In  the  second  case  let  L  (fig.  227)  be  a  luminous  sphere,  and  let 
a  tangent  bn,  be  drawn  externally  to  this  sphere  and  the  sphere  M. 
Assuming  that  this  line  moves  tangentially  round  the  two  bodies,  it 
will  produce  on  the  screen  a  circle,  no,  completely  in  darkness.  If 
now  a  second  straight  line,  bm,  be  drawn  tangentially  on  the  inside 
of  the  two  spheres,  it  will  produce  a  cone  on  the  screen,  the  summit 


-306]  Velocity  of  Light.  299 

of  which  is  at  S,  and  the  base  on  the  screen  is  the  circle  r?n,  which 
is  greater  than  the  circle  no.  The  circular  space  between  the  two 
circumferences  is  neither  entirely  in  the  shadow,  nor  entirely  in  the 
light,  for  it  is  only  illuminated  by  a  part  of  the  body  L  ;  whence 
arises  the  name  penumbra.  Under  ordinary  conditions  in  which 
luminous  bodies  have  a  certain  size,  shadows  are  always  sur- 
rounded by  a  penumbra.  This  decreases  in  intensity  from  the 
centre  towards  the  edges,  and  has  a  greater  extent  the  nearer  the 
luminous  body  is  to  the  body  illuminated,  and  the  more  distant 
the  screen. 


Fig.  227. 

306.  Velocity  of  light. — Light  moves  with  such  a  velocity  that 
at  the  surface  of  the  earth  there  is,  to  ordinary  observation,  no  ap- 
preciable interval  between  the  occurrence  of  any  luminous  pheno- 
menon and  its  perception  by  the  eye.  And,  accordingly,  this  ve- 
locity was  first  determined  by  means  of  astronomical  observations. 
Romer,  a  Danish  astronomer,  in  1675,  first  deduced  the  velocity  of 
light  from  an  observation  of  the  eclipses  of  Jupiter's  first  satellite. 

Jupiter  is  a  planet  round  which  four  satellites  revolve,  as  the 
moon  does  round  the  earth.  This  first  satellite,  e  (fig.. 228),  suffers 
occupation — that  is,  passes  into  Jupiter's  shadow — at  equal  inter- 
vals of  time,  which  are  42  h.  28  m.  36  s.  While  the  earth  moves 
in  that  part  of  its  orbit  nearest  Jupiter,  its  distance  from  that 
planet  does  not  materially  alter,  and  the  intervals  between  two  suc- 
cessive occultations  of  the  satellite  are  approximately  the  same  ;  but 
in  proportion  as  the  earth  moves  away  in  its  revolution  round  the 
sun,  S,  the  interval  between  two  occultations  increases  ;  and  when? 
at  the  end  of  six  months,  the  earth  has  passed  from  the  position 
T  to  the  position  /,  a  total  retardation  of  16  m.  36  s.  is  observed 
between  the  time  at  which  the  phenomenon  is  seen  and  that  at 


300  .     On  Light.  [306  - 

which  it  is  calculated  to  take  place.     But  when  the  earth  was  in  the 
position  T,  the  sun's   light  reflected  from  the  satellite  e  had   to 


traverse  the  distance  eT,  while  in  the  second  position  the  light  had 
to  traverse  the  distance  et.  This  distance  exceeds  the  first  by 
the  quantity  /T,  for,  from  the  great  distance  of  the  satellite  £,  the 
rays  et  and  el  may  be  considered  parallel.  Consequently,  light 
requires  16  m.  56  s.  to  travel  the  diameter  /T  of  the  terrestrial 
orbit,  or  twice  the  distance  of  the  earth  from  the  sun. 

To  give  some  idea  of  this  enormous  velocity,  it  may  be  remarked 
that  a  cannon  ball  would  require  more  than  seventeen  years  to  tra- 
verse the  distance  from  the  earth  to  the  sun,  while  light  would  re- 
quire 8  minutes  and  18  seconds. 

Spite  of  this  enormous  velocity  of  light,  the  stars  nearest  the 
earth  are  separated  from  it  by  at  least  206,265  times  the  distance  of 
the  sun.  Consequently,  the  light  which  they  send  requires  3^  years 
to  reach  us.  Those  stars  which  are  only  visible  by  means  of  the 
telescope,  are  possibly  at  such  a  distance  that  thousands  of  years 
would  be  required  for  their  light  to  reach  our  planetary  system.  We 
may  hence  form  an  idea  of  the  immensity  of  the  heavens,  and  how 
small  is  our  globe  in  comparison  with  this  infinity. 

307.  Intensity  of  light.  Law  of  its  decrease.  Photometer. 
— The  intensity  of  a  source  of  light,  that  is,  the  energy  of  its  illu- 
minating power,  is  measured  by  the  quantity  of  light  which  it  sends 
on  a  given  surface  ;  for  example,  a  screen  a  yard  square.  From 
the  property  which  luminous  rays  have  of  diverging,  this  quantity 
of  light,  this  intensity,  decreases  rapidly  as  the  illuminated  body  is 
removed  from  the  luminous  body.  It  maybe  shown  by  geometrical 
considerations,  that  the  intensity  of  light  is  inversely  as  the  square 
of  the  distance  ;  that  is,  that  when  the  distance  of  an  illuminated 
body  from  the  source  of  light  is  doubled,  it  receives  one-fourth  the 


-307] 


Intensity  of  L  ight.     Photometer. 


301 


amount  of  light  ;    at  three  times  the  distance,  one-ninth,  and  so 
forth. 

This  law  may  be  demonstrated  by  the  aid  of  an  apparatus  called 
a  pliotoineter,  from,  two  Greek  words  which  signify  measure  of  light. 
It  consists  of  a  ground  glass  screen  A,  fixed  vertically  on  a  wooden 
base  (fig.  229).  In  front  of  this  screen  is  an  opaque  rod  B,  beyond 
which  are  the  sources  of  light  to  be  compared,  in  such  a  manner  that 
the  shadows  of  the  rod  form  on  the  screen.  Now  it  will  be  observed, 
that  when  the  two  sources  have  the  same  illuminating  power,  the 
depth  of  the  shadows  is  the  same  :  but  if  one  of  the  sources  of 


Fig.  229. 

light  is  more  powerful  than  the  other,  the  corresponding  shadow  is 
deeper  ;  and  in  order  that  the  shadows  be  of  equal  intensity,  the 
more  powerful  light  must  be  removed  further  away. 

These  details  being  premised,  the  law  of  the  decrease  of  light 
may  be  demonstrated  as  follows  :  In  a  darkroom,  a  candle  is  placed 
at  any  distance  from  the  photometer,  a  yard  for  instance  ;  and  then, 
at  double  the  distance,  four  of  the  same  kind  of  candles  are  placed 
in  the  same  line,  in  the  direction  of  the  opaque  rod.  The  two 
shadows  on  the  screen  will  then  be  found  to  have  exactly  the  same 
depth ;  which  shows  that,  at  two  yard's  distance, four  candles  have  no 
more  illuminating  power  than  one  candle  at  a  distance  of  one  yard ; 


302 


On  Light. 


[307- 


from  which  it  is  concluded  that  each  of  them,  at  double  the  dis- 
tance, has  one  quarter  the  illuminating  power.  It  may  also  be 
shown  in  the  same  manner,  that  nine  candles,  at  three  yards  only, 
have  the  same  illuminating  power  as  one  at  a  yard,  and  so  forth, 
which  proves  the  law. 

It  is  important  to  observe,  that  it  is  in  consequence  of  the  diver- 
gence of  luminous  rays  that  light  decreases  as  the  distance  increases. 
This  decrease  does  not  obtain  in  the  case  of  parallel  rays  :  their 
lustre  would  be  the  same  at  all  distances,  where  it  not  for  the  ab- 
sorption which  takes  place  in  even  the  most  transparent  media. 


CHAPTER   II. 

REFLECTION   OF   LIGHT.      MIRRORS. 

308.  Xiaws  of  the  reflection  of  light. — When  a  ray  of   light 
meets  a  polished  surface,  it  is  not  destroyed  by  this  obstacle  :  but 


Fig.  230. 


-3081 


Reflection  of  Light. 


303 


bounds  off  from  it,  changing  its  direction,  and  this  phenomenon  is 
termed  the  reflection  of  light.  Thus,  if  through  a  hole  in  the  shutter 
of  a  dark  room,  a  pencil  of  the  sun's  rays,  CD,  be  allowed  to  enter, 
and  it  be  received  on  a  plane  mirror,  this  pencil  is  reflected  in  the 
direction  DB,  and  forms  on  the  ceiling  an  image,  the  shape  of  which 
will  be  discussed  in  speaking  of  the  camera  obscura. 

As  in  speaking  of  the  reflection  of  calorific  rays  (210)  the  ray  CD 
is  the  incident  ray,  BD  is  the  reflected  ray,  and  the  straight  line  AD 
at  right  angles  to  the  mirror  is  the  normal.  Lastly,  the  angles  CD  A 
and  ADB  are  called  re- 
spectively the  angles  of  in- 
cidence and  the  angles  of 
reflection. 

The  reflection  of  light  is 
governed  by  the  following 
two  laws,  which,  as  we  have 
seen,  also  prevail  for  heat  : — 

I.  The  angle  of  reflection 
is  equal  to  the  angle  of  inci- 
dence. 

II.  The  incident  and  the 
reflected  ray  are  both  in  the 
same  plane,    which    is  per- 

.  pendicnlar  to   the  reflecting 
surface. 

First  proof.  The  two 
laws  may  be  demonstrated 
by  the  apparatus  represented 
in  fig.  231.  It  consists  of  a 
graduated  circle  in  a  vertical 
plane,  on  three  levelling 
screws.  Two  brass  slides,  I 
and  K,  move  round  the  circumference. 


Fig.  231. 


They  support  two  small 
tubes  i  and  c,  directed  exactly  towards  the  centre,  and  intended  to 
give  passage  respectively  to  the  incident  and  reflected  rays.  On  the 
slide  I  there  is,  moreover,  a  small  mirror  M,  which  can  be  inclined 
at  will.  The  zero  of  the  graduation  is  at  A,  and  extends  to  90 
degrees  on  each  side. 

These  details  being  known,  the  slide  I  having  been  more  or  less 
removed  from  zero,  the  mirror,  M,  is  inclined  so  that  a  luminous 
ray,  S,  after  having  been  reflected  on  this  mirror,  shall  pass  through 


304  On  Light.  [308- 

the  tube  /,  and  fall  upon  a  second  mirror,  m,  arranged  horizontally 
in  the  centre  of  the  circle  :  there  the  luminous  ray  is  reflected 
a  second  time,  and  takes  the  direction  mE.  The  slide  K  is  then 
removed  to  or  from  A,  until  the  eye  being  placed  at  E,  the  reflected 
ray,  mE,  is  received  through  the  tube  c.  If,  now,  the  number  of 
degrees  contained  in  the  arcs  AB  and  AC  be  read  off,  there  will  be 
found  to  be  exactly  equal.  Hence  the  angles  of  incidence  "and  of 
reflection  ¥>ma  and  amC,  measured  by  their  arcs,  are  equal,  which 
verifies  the  first  law. 

The  second  law  is  also  verified ;  for,  in  the  construction  of  the 
apparatus,  care  is  taken  that  the  axes  of  the  tubes,  i  and  c,  are  in 
one  and  the  same  plane  parallel  to  that  of  the  graduated  circle,  and 
therefore  perpendicular  to  the  surface  of  the  small  mirror  ;/*,  and 
containing  the  normal  ma. 

In  the  above  drawing  the  direction  in  which  light  is  propagated 
is  represented  by  arrows  ;  the  same  will  be  the  case  with  all  optical 
diagrams,  which  we  shall  have  occasion  to  introduce. 

309.  The  reflection   in  light  is  never  complete. — The  light 
which  falls  upon  a  body  is  never  completely  reflected ;  a  certain 
portion  is  always  extinguished,  absorbed  by  the  reflecting  surface. 
If  we  represent  by  100  the  quantity  of  incident  light,  the  reflected 
portion  will  be  80,  90,  95,  according  to  the  nature  and  degree  of 
polish  of  the  reflecting  body  ;  but  it  will  never  amount  to  100. 

The  best  reflectors  are  polished  metals,  especially  if  they  are 
white  like  mercury,  and  silver.  Black  bodies  reflect  no  light. 
Translucent  substances  reflect  a  small  quantity,  and  absorb  more  or 
less  according  to  their  thickness,  while  they  transmit  the  remainder. 
This  is  what  takes  place  with  air,  water,  glass,  and  all  transparent 
media. 

For  one  and  the  same  substance  the  quantity  of  reflected  light 
increases  not  only  with  the  degree  of  polish,  but  with  the  obliquity 
of  the  incident  ray.  For  instance,  if  a  sheet  of  white  paper  be 
placed  before  a  candle,  and  be  looked  at  very  obliquely,  an  image  of 
the  flame  is  seen  by  reflection,  which  is  not  the  case  if  the  eye  re- 
ceives less  oblique  rays. 

The  intensity  of  the  reflection  varies  with  different  bodies,  even 
when  the  degree  of  polish  and  the  angle  of  incidence  are  the  same. 
It  also  varies  with  the  nature  of  the  medium  which  the  light  is 
traversing  before  and  after  reflection.  Polished  glass  immersed  in 
water  loses  a  great  part  of  its  reflecting  power. 

310.  Irregular  reflection.     Diffused  or  scattered  light. — The 


-310]  Irregular  Reflection.  305 

reflection  from  the  surfaces  of  polished  bodies,  the  laws  of  which 
have  just  been  stated,  is  called  the  regular  or  specular  reflection  : 
from  a  Latin  word  signifying  mirror  :  but  the  quantity  thus  reflected 
is  less  than  the  incident  light.  The  light  incident  on  an  opaque 
body  is  separated,  in  fact,  into  three  parts  ;  one  is  reflected  regularly, 
another  irregularly,  that  is,  in  all  directions  ;  while  a  third  is  ex- 
tinguished, or  absorbed  by  the  reflecting  body. 

Thus,  if  in  the  experiment  represented  in  fig.  230,  the  beam,  CD, 
be  caught  on  an  unpolished  surface  instead  of  on  a  mirror,  not  only 
will  it  be  seen  in  the  direction  DB,  corresponding  to  regular  reflec- 
tion, but  it  will  be  seen  in  all  positions  in  the  darkroom  :  whence  it 
is  concluded  that  light  is  reflected  in  all  directions  and  under  all 
obliquities  ;  which  is  apparently  contrary  to  the  laws  of  reflection. 

This  irregularly  reflected  light  is  called  scattered  or  diffused  light : 
it  is  that  which  makes  bodies  visible  ;  it  has  its  origin  in  the  struc- 
ture of  bodies  themselves,  which,  from  their  roughness,  present  an 
infinity  of  small  facettes  variously  inclined,  and  which  reflect  light 
in  all  directions. 

Diffused  light  plays  an  important  part  in  the  phenomena  of 
vision.  For  while  luminous  bodies  are  visible  of  themselves,  opaque 
bodies  are  only  so  in  consequence  of  the  diffused  light  which  they 
send  in  all  directions.  Thus  when  we  look  at  a  piece  of  furniture, 
a  table,  or  a  flower,  it  is  the  diffused  light  reflected  on  all  sides,  and 
in  all  directions  by  the  object,  which  enables  us  to  see  them  in  what- 
ever direction  we  may  be  placed  in  reference  to  the  light  which 
illuminates  them.  When  luminous  bodies  only  reflect  light  regu- 
larly, it  is  not  them  we  see,  but,  acting  like  mirrors,  they  only  give 
us  the  image  of  the  luminous  body  whose  light  they  send  towards 
us.  If,  for  example,  a  beam  of  the  sun's  light  falls  on  a  well- 
polished  mirror  in  a  dark  room,  the  more  perfectly  the  light  is  re- 
flected the  less  visible  is  the  mirror  in  the  different  parts  of  the 
room.  The  eye  does  not  perceive  the  image  of  the  mirror,  but  that 
of  the  sun.  If  the  reflecting  power  of  the  mirror  be  diminished  by 
sprinkling  on  it  a  light  powder,  the  sun's  image  becomes  feebler, 
and  the  mirror  is  visible  from  all  parts  of  the  room.  Perfectly 
smooth  polished  reflecting  surfaces,  if  such  there  were,  would  be 
invisible,  and  absolutely  non-reflecting  surfaces  would  also  appear 
all  equally  black,  and  would  be  confounded  with  each  other.  Two 
bodies,  one  white  and  the  other  black,  placed  in  darkness,  are  quite 
invisible,  for  that  which  is  white,  not  receiving  any  light,  can  re- 
flect none. 


306  On  Light.  [310- 

It  is  the  diffused  light  reflected  by  the  air,  by  the  clouds,  by  the 
ground  which  illuminates  our  rooms  and  all  bodies  not  directly 
exposed  to  the  sun's  rays  ;  and  the  more  diffused  light  a  body  sends 
towards  us,  the  more  precisely  can  we  distinguish  it.  From  the 
inside  of  our  rooms  we  well  see  external  objects,  for  they  are  power- 
fully illuminated  ;  but  from  the  outside  we  only  see  confusedly  in 
the  interior  of  apartments  the  objects  found  there,  for  they  receive 
but  little  light. 

311.  Direction  in  which  we  see  bodies. — Whenever  a  pencil 
of  light  passes  in  a  straight  line  from  a  body  to  our  eye,  we  see 
it  exactly  as  it  is  ;  but  if  in  consequence  of  reflection,  or  any  other 
cause,  the  pencil  of  light  is  deviated  in  its  route,  if  it  ceases  to  come 
to  us  in  a  straight  line,  we  no  longer  see  the  body  in  its  proper 


Fig.  232. 

place,  but  in  the  direction  of  tlie  luminous  pencil  'at  the  moment  it 
enters  the  eye.  Thus  if  the  pencil  AB  is  deflected  at  B  (fig.  232),  and 
takes  the  direction  BC,  the  eye  does  not  see  the  point  A  at  A  but  at 
#,  in  the  prolongation  of  CB. 

This  principle  is  general,  and,  though  very  simple,  well  deserves 
the  attention  of  the  reader,  for  on  it  are  based  the  numerous  effects 
of  vision  which  mirrors  and  lenses  present. 

MIRRORS. 

312.  iviirrors.  Images. — Mirrors  are  bodies  with  polished 
surfaces,  which  show  by  reflection  objects  presented  to  them.  The 
place  at  which  objects  appear  is  their  image.  According  to  their 
shape,  they  are  divided  into  plane  and  curved  mirrors. 

We  have  an  example  of  a  plane  mirror  in  the  looking  glasses 
which  adorn  our  apartments.  In  these  mirrors  it  is  not  the  glass 
which  reflects  light  in  sufficient  quantity  to  give  neat  and  well 
denned  images  ;  it  is  a  metallic  layer  on  the  back  of  the  glass. 
This  layer  is  an  amalgam  of  tin,  that  is,  an  alloy  of  this  metal  with 
mercury.  The  glass  only  has  the  effect  of  giving  the  metal  the 
necessary  polish,  and  of  preserving  it  from  external  agencies  which 
tend  to  tarnish  it. 

Metal  mirrors  are  also  constructed  of  gold,  silver,  steel,  tin. 


-313]       Formation  of  Images  in  Plane  Mirrors.         307 

They  have  all  the  defect  of  tarnishing  on  contact  with  the  air ;  yet 
they  were  in  frequent  use  among  the  Romans.  We  cannot  go 
back  to  the  origin  of  mirrors.  The  first  was  doubtless  the  surface 
of  clear  water.  Those  of  metal  appear  to  be  of  high  antiquity,  for 
mention  is  made  in  Exodus  of  a  bronze  ewer,  made  by  Moses  with 
the  mirrors  offered  him  by  the  Israelitish  women. 

313.  Formation  of  images  in  plane  mirrors. — Plane  mirrors 
are  those  whose  surface  is  plane  ;  such,  for  example,  are  the  pier 
glasses  which  adorn  the  chimney-pieces  of  our  rooms.  To  under- 


Fig.  233. 

stand  the  formation  of  images  in  these  mirrors,  let  us  first  con- 
sider the  case  in  which  a  small  object  is  placed  in  front  of  such  a 
mirror  ;  for  instance,  the  flame  of  a  candle  (fig.  233).  A  divergent 
pencil  of  light  emitted  by  this  flame  and  falling  on  the  mirror  is 
reflected  there,  as  shown,  in  fig.  233.  But  it  follows  from  the 
laws  of  reflection,  that  each  ray  of  this  pencil  retains,  in  reference  to 
the  mirror,  the  same  obliquity  as  it  had  before  ;  whence  it  follows 
that  the  reflected  rays  have  the  same  divergence  in  reference  to 


308 


On  Light. 


[313- 


each  other  as  the  incident  rays.  Hence  if  we  imagine  the  reflected 
pencil  prolonged  behind  the  mirror,  all  the  rays  composing  it  will 
coincide  in  the  same  point.  But  as  we  always  see  objects  in  the 
direction  the  luminous  rays  have  when  they  reach  us  (311),  it  follows 
that  the  eye  which  receives  the  reflected  pencil  should  see  the  flame 
of  the  candle  just  in  the  place  where  the  prolongation  of  these 
reflected  rays  coincide.  There,  in  fact,  is  produced  the  image  of 
this  flame  as  seen  in  fig.  233. 

If  how,  instead  of  supposing  a  very  small  object  placed  in  front 
of  the  mirror,  we  consider  a  body  of  any  dimensions,  in  order  to 
understand  the  formation  of  its  image  we  need  do  no  more  than 


Fig.  234. 

apply  to  each  of  its  parts  w;hat  has  been  said  in  reference  to  a 
single  luminous  point  ;  for  instance,  in  fig.  234,  which  represents  a 
person  in  front  of  a  mirror,  the  rays  from  the  forehead,  for  instance, 
are  reflected  from  the  mirror  and  return  to  the  eye,  producing  an 
image  of  the  forehead.  In  like  manner  the  rays  from  the  chin 
being  reflected  from  the  mirror  reach  the  eye  as  if  they  proceeded 
from  the  chin  of  the  image,  and  so  on  with  all  parts  of  the  face  ; 
hence  the  illusion  which  makes  us  see  our  image  on  the  other  side 
of  the  mirror. 

<     314.  Nature    of  the   images    in    plane  mirrors.     Real   and 
virtual  images. — If,  while  looking  in  a  mirror,  we  raise  the  right 


-315]     Multiple  Images  Formed  by  Glass  Mirrors.      309 

hand,  it  is  the  left  which  seems  raised  in  the  mirror  ;  and  if  we 
raise  the  left  hand  the  right  seems  raised.  We  should  falsely  ex- 
press this  transposition  of  the  parts  of  the  image  in  reference  to 
the  object  if  we  merely  say  that  the  image  was  reversed  ;  if  it  were 
nothing  else  than  the  object  reversed,  in  raising  the  right  hand  the 
image  should  also  raise  the  right  hand,  while  it  really  is  the  left 
which  is  raised. 

This  special  equality  which  exists  between  an  object  and  its 
image  is  expressed  by  saying,  that  the  image  is  symmetrical  in 
reference  to  the  object ;  that  is,  that  any  point  of  the  image  is 
arranged  behind  the  mirror  in  identically  the  same  manner  as  the 
corresponding  point  of  the  object  in  front.  For  it  may  be  shown 
by  geometrical  considerations,  that  these  two  points  are  equidistant 
from  the  mirror,  and  on  the  same  right  line,  which  is  at  right  angles 
.  to  the  surface.  From  the  respective  distance  and  position  of  the 
different  parts  of  the  object  and  of  its  image,  it  is  concluded  that 
the  latter  is  of  the  same  magnitude  as  it,  and  equidistant  from  the 
mirror. 

Lastly,  images  formed  in  plane  mirrors  are  virtual,  by  which  we 
mean,  that  they  have  no  real  existence,  and  are  only  an  illusion  of 
the  eyes.  For  in  fig.  233  as  well  as  in  fig.  234  the  light,  as  it  does 
not  pass  behind  the  mirror,  cannot  form  any  image  there,  and  that 
which  we  see  has  no  existence  ;  this  is  expressed  by  the  word 
virtual  as  opposed  to  actual  or  real.  Virtual  images  are  only  an 
optical  illusion  ;  but  we  shall  soon  see  that,  in  concave  mirrors  and 
in  lenses,  real  images  are  produced  which  can  be  received  on 
screens  ;  this  is  not  the  case  with  virtual  images. 

We  may  thus  sum  up  what  we  have  said  :  images  in  plane 
mirrors  are  symmetrical  in  reference  to  the  object,  of  the  same  magni- 
tude, at  the  same  distance  on  the  other  side  of  the  mirror,  and  are 
virtual. 

315.  Multiple  images  formed  by  glass  mirrors. — Metallic 
mirrors  which  have  but  one  reflecting  surface  only  give  one  image ; 
it  is  different  with  glass  mirrors,  the  two  surfaces  of  which  reflect 
though  to  an  unequal  extent.  For  if  we  apply  any  object,  the 
point  of  a  pencil,  for  instance,  against  a  thick  piece  of  polished 
glass,  at  first  when  it  is  looked  at  obliquely  a  very  feeble  image  is 
seen  in  contact  with  it ;  then,  beyond  it,  another  and  far  more  in- 
tense one.  The  first  image  is  due  to  the  light  reflected  from  the 
anterior  surface  of  the  plate  ;  that  is,  on  the  glass  itself,  while  the 
second  is  due  to  the  light  which  penetrating  into  the  glass,  is  re- 


3io 


On  Light. 


[315- 


fleeted  from  the  layer  of  metal  by  which  the  posterior  face  is 
covered.  The  difference  in  intensity  of  the  two  images  is  readily 
explained  ;  glass  being  very  transparent,  only  a  small  quantity  of 
light  is  reflected  from  the  rirst  face  of  the  mirror,  which  gives  the 
least  intense  image  ;  while  the  greater  part  of  the  incident  light 
passing  into  the  mass  is  reflected  from  the  surface  of  the  metal,  and 
gives  the  most  luminous  image. 

The  above  experiment  furnishes  a  simple  means  of  measuring 
the  thickness  of  a  glass  mirror.  For  the  more  intense  image  should 
appear  behind  the  layer  of  metal  at  the  same  distance  as  the  point 
of  the  pencil  in  front ;  and  it  follows  thence,  that  the  distance 
between  the  point  of  the  pencil  and  the  point  of  its  image  is  double 
the  thickness  of  the  mirror.  If  this  distance  seems  to  be  the  eighth 
of  an  inch,  it  will  be  concluded  that  the  real  thickness  is  —th  of  an 
inch. 

The  double  reflection  from  mirrors  is  prejudicial  to  the  sharpness 
of  the  images,  so  that,  in  scientific  observations,  metallic  mirrors  are 
preferred  to  glass  ones. 


Fig.  235. 

316.  Reflection  from  transparent  bodies. — We  have  seen  that 
glass,  spite  of  its  transparency,  reflects  a  sufficient  amount  of  light 
to  give  images,  which,  though  feeble,  are  distinct.  The  same  is  the 
case  with  water  and  other  transparent  liquids.  Thus,  on  the  borders 


—317]     Multiple  Images  in  Parallel  Glass  Mirrors.     3 1 1 

of  a  pool,  we  see  formed  in  the  water  the  reversed  image  of  objects 
on  the  opposite  bank.  We  say  reversed  image,  so  as  to  express  the 
appearance  ;  but  more  strictly  we  should  say  symmetrical,  from 
what  we  have  before  said  (314). 

Fig"  235  represents  the  phenomenon  of  reflection  from  the 
surface  of  water  ;  it  shows  how  the  reflected  rays,  reaching  the  eye 
in  an  upward  direction,  reproduce  the  image  of  objects  situated 
above  the  water,  just  as  they  would  if  reflected  from  a  horizontal 
mirror. 

317.  Multiple  images  in  parallel  or  inclined  glass  mirrors. — 
When  a  source  of  light  is  placed  between  two  plane  parallel 
mirrors,  we  observe  a  series  of  images  the  brightness  of  which 
gradually  decreases. 


Fig.  236. 

These  images  are  due  to  successive  reflections  on  the  two  mirrors. 
Thus,  let  M  and  N  fig.  236  be  the  sections  of  the  two  mirrors,  A  a 
luminous  body,  and  o  the  eye  of  the  observer.  This  latter  receives 
the  rays  which  come  directly  from  the  object  A,  and  in  addition 
the  following  pencils  :  i.  the  ray  A&?,  which  after  a  single  reflection 
gives  the  first  image  a  ;  ii.  the  ray  Aufo,  which  after  two  reflections 
furnishes  the  second  image  a"  ;  iii.  the  ray  kefgo,  which  is  reflected 
three  times,  and  produces  a  third  image  a'  and  so  on  for  rays  which 


312  On  Light.  [317- 

undergo  four,  five,  or  six  reflections.  The  number  ot  images  is 
theoretically  infinite,  but  in  practice  it  is  limited,  for  as  light  is  never 
completely  reflected  at  each  incidence  (309)  the  images  successively 
lose  their  lustre  and  finish  by  disappearing  entirely. 

In  order  not  to  complicate  the  figure,  only  those  rays  are  given 
which  fall  at  first  on  the  mirror  M;  but  the  same,  construction 
should  be  repeated  for  the  mirror  N,  which  would  double  the 
number  of  images. 


If  the  mirrors,  instead  of  being  parallel,  make  an  angle  with 
each  other,  the  number  of  images  is  less.  Fig.  237  represents  the 
case  in  which  they  form  a  right  angle  ;  three  images,  a,  a',  a",  are 
then  formed  ;  the  first  two  after  a  single  reflection  ;  the  third  after 
two  reflections.  If  the  angle  is  one  of  60  degrees,  there  are  five 
images. 

The  Kaleidoscope,  which  consists  of  three  glass  mirrors  enclosed 
in  a  pasteboard  tube  at  an  angle  of  60°,  and  the  Debuscope  of  two 
mirrors  at  an  angle  of  60°,  are  well  known  applications  of  this  pro- 
perty of  reflection  from  inclined  mirrors. 


CURVED   MIRRORS. 

318.  Concave  mirrors. — There  are  many  kinds  of  curved 
mirrors  ;  those  most  in  use  are  called  spherical  mirrors,  from  their 
curvature  being  that  of  a  sphere.  They  may  be  either  of  metal 
or  of  glass,  and  are  either  concave  or  convex:,  according  as  the 
reflection  is  from  the  internal  or  the  external  face  of  the  mirror. 


-319]  Focus  of  Concave  Mirrors.  313 

A  curved  watch-glass,  seen  from  above,  gives  an  idea  of  a  convex 
mirror,  especially  if  it  is  covered  by  a  coating  of  metal  on  the  in- 
side ;  the  same  glass  coated  externally  and  seen  from  the  inside 
becomes  a  concave  mirror. 

We  shall  first  investigate  concave  mirrors,  and,  to  facilitate  the 
investigation,  will  first  consider  what  is  called  a  section  ;  that  is, 
the  figure  obtained  by  cutting  it  into  two  equal  parts.  Let  MN 
be  the  section  of  a  spherical  mirror,  and  C  the  centre  of  the  corre- 
sponding sphere.  In  reference  to  the  sphere  this  point  is  called 
the  centre  of  curvature  ;  the  point  A  is  the  centre  of  the.  figure. 
The  infinite  right  line,  ACX,  which  passes  through  A  and  C,  is  the 
principal  axis  of  the  mirror  :  any  right  line,  /CW,  which  simply 
passes  through  the  centre  C,  and  not  through  the  centre  of  figure 
A,  is  a  secondary  axis.  The  angle  MCN,  formed  by  joining  the 


centre  and  extremities  of  the  mirror,  is  the  aperture.  A  principal 
or  meridional  section  is  any  section  made  by  a  plane  through  its 
principal  axis.  In  speaking  of  mirrors  those  lines  alone  will  be 
considered  which  lie  in  the  same  principal  section.  There  is  only 
one  principal  axis,  but  the  number  of  secondary  axes  is  unlimited. 

The  theory  of  the  reflection  of  light  from  curved  mirrors  is 
easily  deduced  from  the  laws  of  reflection  from  plane  mirrors,  by 
considering  the  surface  of  the  former  as  made  up  of  an  infinitude 
of  extremely  small  plane  surfaces,  all  equally  inclined  to  each  other 
so  as  to  form  a  regular  spherical  surface.  Thus,  on  this  hypo- 
thesis, when  a  ray  of  light  falls  upon  any  point  whatever  of  a  curved 
mirror,  it  is  really  from  a  small  plane  mirror  that  it  is  reflected ; 
the  reflection  takes  place  then  in  accordance  with  the  laws  already 
laid  down  (308). 

3 1 9.  Focus  of  concave  mirrors.—  The  small  facettes,  of  which 
we  have  assumed  concave  mirrors  to  be  made  up,  being  all  inclined 
towards  a  common  centre,  which  is  the  centre  of  curvature  of  the 


On  Light. 


[319- 


mirror,  it  follows  from  this  obliquity  that  the  rays  reflected  by  these 
mirrors  tend  to  unite  in  a  single  point,  which  is  called  the  focus, 
as  we  have  already  seen  in  the  case  of  heat  (211). 

To  explain  this  property  of  curved  mirrors,  let  SI  be  a  ray  fall- 
ing upon  such  a  mirror  parallel  to  the  axis  AX  (fig.  238).  From 
the  hypothesis  assumed  above,  the  reflection  takes  place  at  I,  on  an 
infinitely  small  plane  mirror.  It  can  be  shown  by  geometrical  con- 
siderations that  the  perpendicular  to  this  small  mirror  is  represented 
by  the  right  line  CI  from  the  centre  C  to  the  point  I.  Hence  the 


Fig    239. 


angle  SIC  represents  the  angle  of  incidence  ;  and  it  we  imagine  on 
the  other  side  of  the  perpendicular,  a  straight  line  IF,  which  makes 
with  CI  an  angle  FIC,  equal  to  CIS,  this  straight  line  will  be  in  the 
direction  of  the  reflected  ray. 

But  when  the  incident  rays  are  parallel  to  the  axis  of  the  mirror, 
as  in  the  above  example,  it  may  be  proved  by  geometrical  con- 
siderations, that  the  point  F,  where  the  luminous  ray  cuts  the  axes, 
is  the  middle  of  AC  ;  that  is,  it  is  equidistant  from  the  centre  and 
the  mirror.  This  property  being  common  to  all  rays  parallel  to 


-320]  Conjugate  Focus.  315 

the  axis,  it  follows  that,  after  reflection,  these  rays  will  all  coincide 
in  the  same  focus,  F,  as  shown  in  the  figure. 

The  focus  described  above,  namely,  that  formed  at  an  equal 
distance  from  the  centre  and  from  the  mirror,  is  called  \^Q  principal 
fociis  ;  it  is  produced  whenever  the  rays  falling  on  the  mirror  are 
parallel  to  its  axis.  An  example  of  this  is  seen  in  fig.  239,  which 
represents  a  pencil  of  solar  light  falling  upon  a  concave  mirror.  If, 
where  the  reflected  rays  tend  to  concentrate  themselves,  a  small 
ground  glass  screen  be  placed,  a  highly  luminous  point  will  appear, 
which  is  the  principal  focus. 

320.  Conjugate  focus.— In  the  preceding  examples  we  have 
considered  the  case  of  pencils  of  parallel  rays,  which  presupposes 
a  luminous  object  at  an  infinite,  or  at  all  events  a  very  great, 
distance.  Let  us  now  consider  the  case  in  which  the  source  of 
light  being  at  a  small  distance,  the  rays  falling  on  the  mirror  are 
divergent,  as  shown  in  fig.  240.  Here  the  reflected  rays  are  con- 


Fig.  240. 

verged,  but  less  so  than  in  figs.  238  and  239,  which  results  from  the 
divergence  of  the  light  in  arriving  on  the  mirror.  Hence  the  point 
where  the  reflected  rays  coincide  is  more  distant ;  instead  of  being 
at  F,  equidistant  from  the  mirror  and  the  centre,  it  is  at  b,  between 
the  points  F  and  C.  This  point,  b,  where  the  rays  coincide,  is 
also  a  focus.  To  distinguish  it  from  the  principal  focus  F,  it  is 
called  the  conjugate  focus,  from  a  Latin  word  meaning  connected ; 
for  there  is  between  the  position  of  the  luminous  point  B,  and  that 
of  this  focus,  this  connection,  that  when  the  luminous  object  is 
at  B,  the  rays  form  their  focus  at  b\  and  that  conversely,  if  the 
luminous  object  is  removed  to  b,  the  reflected  rays  form  their  focus 
at  B. 


316  On  Light.  [320- 

We  have  seen  that  there  is  only  a  single  position  for  the  prin- 
cipal focus,  which  is  at  an  equal  distance  from  the  centre  and  from 
the  mirror  :  this  is  not  the  case  with  the  conjugate  focus,  the 
position  of  which  is  very  variable.  For  suppose  that  in  fig.  240 
the  candle  is  removed  away  from  the  mirror,  as  the  incident  rays 
make  then,  with  the  perpendicular,  cm,  gradually  increasing  angles 
of  incidence,  the  angles  of  reflection,  cmb,  increase  too,  and  the 
focus  b  approaches  the  point  F,  with  which  it  will  ultimately  coin- 
cide, when  the  candle  is  so  far  distant  that  the  incident  rays  are 
virtually  parallel. 

If,  on  the  contrary,  the  candle  is  brought  nearer  the  mirror,  the 
rays  falling  upon  it  make  with  the  perpendicular,  cm,  angles  which 
are  gradually  smaller,  the  angles  of  reflection,  cmb,  decrease  also. 
Hence  the  rays  sent  by  the  mirror  coincide  at  gradually  greater 
distances,  the  focus  b  advances  towards  the  centres;  and  if  the 
candle  comes  nearer  the  point,  so  as  to  coincide  with  it,  the  case 
will  be  the  same  as  the  focus  b ;  so  that  the  candle  and  its  image 
will  coincide  at  c. 

Lastly  ;  if  the  candle  always  approaching  the  mirror  passes 
between  the  centre  and  the  principal  focus  F,  the  conjugate  focus 
b,  continually  removing  from  the  mirror,  passes  on  the  other  side  of 
the  centre,  is  formed  at  a  greater  distance  the  nearer  the  luminous 
body  is  to  the  principal  focus ;  if  the  candle  coincides  with  this 
latter  point,  the  conjugate  focus  forms  at  an  infinite  distance,  and 
the  reflected  rays  become  parallel. 

These  different  effects  of  reflection  are  a  consequence  of  the 
constant  equality  between  the  angle  of  incidence  and  the  angle  of 
reflection.  They  are  very  simply  verified  by  placing  in  a  dark 
room  a  candle  in  front  of  a  concave  mirror  successively  in  various 
positions,  and  then  ascertaining  by  trial  where  the  luminous  focus 
is  formed  on  a  small  screen  of  paper  held  in  the  hand,  and  which  is 
approached  to  or  receded  from  the  mirror. 

321.  Virtual  focus. — After  having  described  the  different  po- 
sitions of  the  point  in  which  the  rays  reflected  by  a  concave  mirror 
coincide,  when  the  luminous  body  is  either  beyond  or  in  the  prin- 
cipal focus,  we  have  to  inquire  what  becomes  of  these  same  rays 
when  the  source  of  light  is  in  any  point,  P,  which  is  nearer  the 
mirror  than  the  principal  focus  (fig.  241).  In  this  case  the  reflec- 
ted rays  form  a  diverging  pencil,  and  cannot  therefore  produce  any 
focus  in  front  of  the  mirror  ;  but  as  regards  the  eye  which  receives 
them,  they  produce  exactly  the  same  effect  as  in  plane  mirrors 


-322]     Formation  of  Images  in  Concave  Mirrors.        3 1 7 

(313);  that  is,  the  eye  receives  exactly  the  same  impression  from  the 
reflected  rays  IM  and  im,  as  if  the  candle  were  placed  behind  the 


Fig.  241. 

mirror  at  the  point/,  where  the  prolongation  of  these  rays  coincide. 
Hence  the  image  of  the  candle  is  seen  at  p,  but  as  the  light  does 
not  penetrate  behind  the  mirror,  this  image  does  not  really  exist  : 
hence  the  focus  which  seems  to  form  at  p  is  called  the  virtual 
focus,  the  expression  being  understood  in  the  same  sense  as  in 
plane  mirrors. 

322.  Formation  of  images  in  concave  mirrors. —  Concave 
mirrors  give  rise  to  two  kinds  of  images,  real  and  virtual.  Their 
formation  is  readily  understood  after  what  has  been  said  respecting 
the  conjugate  and  the  virtual  focus.  We  may  however  remark, 
when  a  luminous  or  illuminated  point  is  situate  on  the  principal 
axis  of  a  mirror,  its  focus,  real  or  virtual,  is  always  formed  on  this 
axis.  This  is  the  case  in  figs.  240  and  241,  but  if  the  luminous 
point  is  on  a  secondary  axis,  the  focus  is  formed  on  this  axis. 
Thus,  if  in  fig.  238  a  candle  were  placed  at  d,  on  the  secondary 
axis,  z'C</,  the  reflected  rays  would  form  their  focus  on  the  line  Cz. 
That  being  admitted,  let  us  see  how  images  are  formed  in  concave 
mirrors. 

Real  image.  If  a  person  places  himself  at  a. certain  distance  in 
front  of  a  concave  mirror,  he  no  longer  sees  himself  erect  and  of 
the  ordinary  size,  as  in  plane  mirrors, '  but  reversed,  and  much 
smaller,  as  shown  in  fig.  242.  To  this  image  is  given  the  name 
real  image,  to  express  that  it  is  not  an  illusion  as  that  seen  in  plane 
mirrors,  but  that  it  has  a  real  existence.  For  it  may  be  caught  on 
a  screen.  If  the  mirror  be  placed  in  front  of  an  object  powerfully 
illuminated,  as,  for  instance,  before  a  building  on  which  the  sun  is 
shining,  and  a  person  places  himself  a  little  on  one  side,  holding  a 
small  white  screen  in  the  position  in  which  the  conjugate  focus 
should  be  formed,  the  pencils  from  the  various  parts  of  the  edifice 


On  Light. 


[322- 


are  reflected  from  the  mirror  and  fall  on  the  screen,  forming  in 
minature  an  image  not  less  remarkable  for  the  colour,  than  for  the 
fidelity  of  the  contours  (fig.  243)  :  it  has  no  other  defect  than  that 
of  being  reversed.  . 

The  formation  of  this  image  is  readily  explained.  For  from 
what  has  been  said  in  reference  to  conjugate  foci  (320),  each  point 
of  the  image  is  the  conjugate  focus  of  the  corresponding  point  of  the 
illuminated  body,  and  is  on  the  same  secondary  axis.  But  as  all 


Fig.  242. 


the  secondary  axes  from  the  various  points  of  this  body  cross  in 
the  centre  of  curvature  of  the  mirror,  it  follows,  as  shown  in  the 
figure,  that  the  rays  emitted  by  the  higher  parts  of  the  body  converge 
towards  the  lower  part  of  the  image,  and  that  conversely  rays  from 
the  foot  unite  on  the  higher  parts  of  this  same  image,  which  explains 
how  it  is  the  latter  is  reversed. 

*  It  is  to  be  observed  that  the  real  image  in  concave  mirrors  is 
not  always  smaller  than  the  object  illuminated,  as  is  the  case  in 
the  above  two  figures  ;  it  may  also  be  larger.  This  is  the  case 
when,  the  object  being  placed  between  the  principal  focus  and  the 


-322] 


Virtual  Image. 


319 


centre  of  curvature,  its  image  is  formed  outside  the  latter,  and  it  is 
then  larger  the  greater  the  distance  at  which  it  is  formed. 

Virtual  image.  The  above  figure  (242)  shows  how,  when  a  person 
is  placed  at  a  certain  distance  in  front  of  a  concave  mirror,  he  sees 
himself  smaller  and  reversed.  If  he  comes  nearer,  there  is  a 
point  at  which  no  image  is  seen.  This  is  the  case  when  he  is 
between  the  centre  and  the  principal  focus,  for  the  image  is  thus 
formed  behind  the  observer.  If  he  is  in  the  principal  focus  itself, 
there  is  no  image.  For  we  know  (320)  that  the  luminous  rays 


proceeding  from  this  focus,  after  being  reflected  from  a  concave 
mirror  produce  a  parallel  luminous  pencil :  hence,  as  the  rays 
coincide  neither  behind  nor  in  front  of  the  mirror,  they  cannot 
give  rise  to  any  image.  But,  approaching  the  mirror,  the  image 
suddenly  reappears,  and  instead  of  being  smaller  and  reversed  as 
it  was,  is  now  erect  and  much  enlarged,  as  in  fig.  244.  This  is  the 
virtual  image. 

To  account  for  the  formation  of  this  image  we  must  recall  what 
has  been  said  about  the  virtual  focus  (321)  :  First,  that  it  is  only 


320 


On  Light. 


[322^ 


formed  as  long  as  the  luminous  or  illuminated  object  is  between 
the  principal  focus  and  the  mirror  ;  second,  that  the  virtual  focus, 

or,  what  is  the  same  thing,  the 
virtual  image  of  any  point  of  the 
object,  is  behind  the  mirror  on 
the  secondary  axis  which  passes 
through  this  point.  Hence,  the 
head  of  the  observer  being  placed 
between  the  mirror  and  the  princi- 
pal focus  (fig.  245),  all  rays  from 
any  point,  a,  of  the  face  return  to 
the  eye  after  reflection,  as  if  they 
proceeded  from  the  point,  A,  where 
the  prolongations  of  the  reflected 
rays  coincide  on  the  secondary 
axis,  CaA.  In  like  manner,  rays 
from  the  point  b  return  to  the  eye, 
as  if  they  were  emitted  from  the 
point  B,  which  is  on  the  pro- 
longation of  the  secondary  axis,  C^B.  The  eye  sees,  therefore, 
at  AB,  an  erect  and  enlarged  image. 


Fig.  244. 


Fig-  245- 


323.    Formation   of  images   in    convex   mirrors. — We    have 
already  seen  that  convex  mirrors  are  spherical  mirrors,  which  re- 


-324] 


Applications  of  Mirrors. 


321 


fleet  light  from  their  external  surface,  that  is,  on  their  bulbed  side. 
Whatever  the  distance  of  a  luminous  or  illuminated  object  placed 
in  front  of  these  mirrors,  we 
never  obtain  any  other  than 
a  virtual  image  situated  on 
the  other  side  of  the  mirror, 
always  erect,  and  smaller 
than  the  object.  This  may 
be  verified  by  looking  in  a 
mirror  of  this  kind,  as  repre- 
sented in  figure  246.  The 
formation  of  this  image  can 
be  easily  explained  by  an 
inspection  of  figure  247.  It 
is  here  smaller  than  the  ob- 
ject, for  it  is  nearer  than 
the  latter  to  the  point  where 
the  secondary  axes  coincide, 
while  the  reverse  is  the  case 
with  the  formation  of  the  vir- 
tual image  in  concave  mirrors.  Flg'  24(5' 

324.  Applications  of  mirrors. — The  applications  of  plane 
mirrors  in  domestic  economy  are  well  known.  Mirrors  are  also 
frequently  used  in  physical  apparatus  for  sending  light  in  a  certain 
direction.  The  solar  light  can  only  be  sent  in  a  constant  direction 


Fig.  247. 

by  making  the  mirror  movable.  It  must  have  a  motion  which 
compensates  for  the  continual  change  in  the  direction  of  the  sun's 
ra)  s  produced  by  the  apparent  diurnal  motion  of  the  sun.  This 
result  is  obtained  by  means  of  a  clock-work  motion,  to  which  the 

y 


322 


On  Light. 


[324- 


mirror  is  fixed,  and  which  causes  it  to  follow  the  course  of  the  sun. 
This  apparatus  is  called  the  heliostat.  The  reflection  of  light  is 
also  used  to  measure  the  angles  of  crystals  by  means  of  the  instru- 
ments known  as  reflecting  goniometers. 

Concave  spherical  mirrors  are  also  often  used.  They  are  some- 
times applied  for  magnifying  mirrors,  as  in  a  shaving  mirror.  They 
have  been  employed  for  burning  mirrors,  and  are  still  used  in  tele- 
scopes, as  they  only  give  one  image.  They  also  serve  as  reflectors, 
for  conveying  light  to  great  distances,  by  placing  a  luminous  object 
in  their  principal  focus.  For  this  purpose,  however,  parabolic 
mirrors  are  preferable. 


CHAPTER   III. 


REFRACTION    OF   LIGHT. 

325.  Phenomenon  of  refraction. — When  a  ray  of  light  passes 
more  or  less  obliquely  from  one  transparent  medium  into  another  ; 

for  instance,  from  air  into  water,  or 
from  air  into  glass,  it  undergoes  a  de- 
flection from  the  straight  line  in  which 
it  proceeds,  as  seen  in  fig.  248,  which 
represents  a  ray  of  light  passing  from 
air  into  water.  This  change  in  direc- 
tion is  called  refraction,  from  a  Latin 
word  meaning  broken  ;  for  the  ray  is, 
in  fact,  broken  at  the  point  A,  where . 
it  passes  from  the  direction  LA  to  the 
direction  AK. 

The  ray,  LA,  is  called  again  the  incident  ray  ;  AK  is  the  re- 
fracted ray  ;  the  perpendicular,  BC,  drawn  at  the  point  of  incidence, 
A,  of  the  surface,  MN,  which  separates  the  two  media,  is  called  the 
normal  or  perpendicular ;  lastly,  the  angle.  BAL,  is  called  the 
angle  of  incidence ;  and  the  angle,  CAK,  the  angle  of  refraction. 
If  the  angle  of  incidence  is  null,  that  is,  if  the  incident  ray  coincides 
with  the  normal,  the  same  is  the  case  with  the  refracted  ray,  and 
light  then  travels  in  a  straight  line. 


Fig.  248. 


-328]  Experimental  Proofs  of  Refraction.  323 

326.  Ziaws  of  refraction. — When  a  ray  of  light  is  refracted  in 
passing  from  one  medium  into    another  of  a  different  refractive 
power  the  following  laws  prevail : 

I.  Whatever  the  obliquity  of  the  incident  ray,  the  ratio  which 
the  sine  of  the  incident  angle  bears  to  the  sine  of  the  angle  of  refrac- 
tion, is  constant  for  the  same  two  media,  but  varies  with  different 
media. 

I 1.  The  incident  and  the  refracted  ray  are  in  the  same  plane, 
which  is  perpendicular  to  the  surface  separating  the  two  media. 

These  laws  may  be  -understood  by  reference  to  the  adjoined 
figure  (249),  in  which  the  ray,  LA,  passes  from  air  into  water.  If 
from  the  point  of  incidence,  with  a  radius 
equal  to  unity,  a  circle  be  described,  and 
from  the  points,  m  and  p,  where  it  cuts 
the  incident  and  refracted  rays,  perpen- 
diculars, mn  and  pq,  are  drawn  to  the 
normal,  BC,  the  former  is  called  the  sine 
of  the  angle  of  incidence,  and  the  second 
the  sine  of  the  angle  of  refraction. 

It  is  the  ratio  of  these  sines,  these  per- 
pendiculars, which  is  constant ;  that  is, 
that  pq,  for  instance,  being  three-quarters  FIs-  249- 

of  mn,  if  the  angle  of  incidence  diminishes  or  increases,  the  angle 
of  refraction  does  so  too,  but  the  sine  of  the  latter  will  always  be 
three-quarters  of  the  sine  of  the  former. 

This  constant  ratio  is  called  the  refractive  index,  or  index  of  re- 
fraction ;  its  value  varies  with  different  transparent  media. 

327.  Refracting:  substances. — When  a  ray  of  light  is  refracted 
in  passing  from  one  medium  into  another,  sometimes  it  approaches 
the  normal,  forming  an  angle  of  refraction,  which  is  less  than  the 
angle  of  incidence,  as  is  the  case  in  the  above  figure  ;  sometimes, 
on  the  contrary,  it  is  deflected  away,  forming  an  angle  of  refrac- 
tion, which  is  greater  than  the  angle  of  incidence.      In  the  first 
case  the  second  medium  is  said  to  be  more  refringent  or  refrac- 
tive than  the  first,  and  in  the  second  case  it  is  less  so. 

Among  the  most  refringent  bodies  are  water,  alcohol,  ether,  the 
fat,  oils,  etc.  Diamond  is  the  most  refringent  of  all  bodies.  Gases 
are  less  refringent  than  water ;  their  refracting  power  is  increased 
by  condensing  them,  that  is,  by  increasing  their  density. 

328.  Experimental    proofs    of    refraction. — The     deviation 
undergone  by  luminous  rays,  on   passing  from   one   medium  to 

Y  2 


324 


On  Light. 


[328- 


another,  may  be  demonstrated  by  numerous  experiments.  Thus, 
if  in  a  dark  room  a  pencil  of  the  sun's  rays  be  allowed  to  fall  on  a 
glass  vessel  containing  water  (fig.  250),  the  pencil  can  be  very  dis- 
tinctly seen  to  be  broken  as  it  passes  from  air  into  water,  especially 
if  some  light  powder  has  been  diffused  through  the  air  and  the  water 
so  as  to  make  the  pencil  more  visible  (309). 


Fig.  250. 

Or  let  a  coin  be  placed  at  the  bottom  of  an  opaque  vessel  (fig. 
251),  and  the  eye  be  placed  so  that  the  edge  of  the  vessel  just  inter- 
cepts the  view  of  the  coin.  If,  now,  without  altering  the  position  of 
the  observer,  a  little  water  be  gradually  poured  into  the  vessel,  at 
first  only  the  edges  of  the  coin  will  be  seen,  then  half,  and  finally 
the  entire  piece.  Now  what  has  taken  place  here  ?  Nothing  has 
been  changed  in  the  position  of  the  eye,  or  in  that  of  the  piece  : 
it  is  the  rays  from  the  latter  which  have  changed  their  direction. 
Those  which  were  before  intercepted  by  the  sides  of  the  vessel  are 
so  still  ;  but  rays  which,  before  there  was  water  in  the  vessel,  passed 
a  bove  the  observer's  head,  are  directed  towards  the  eye,  being  re- 


-329] 


Various  Effects  of  Refraction. 


325 


fracted  in  passing  from  water  into  air,  so  as  to  diverge  from  the  per- 
pendicular to  the  surface  of  the  liquid,  as  represented  in  the  figure. 
329.  Various  effects  of  refraction. — Refraction   of  light  pro- 
duces  various  phenomenon, 
the  effect  of  which  is  to  de- 
ceive the  eye  by  making  us 
see    objects    in    other   than 

their  true  positions  ;  thus,  we  *, 

do  not  see  fish  in  the  place 
they  actually  occupy,  but  a 
little  higher,  as  indicated  by 
the  path  of  the  refracted  ray 
in  fig.  252.  It  will  be  under- 
stood that  in  consequence 
of  the  same  phenomena  we 

see  the  bottom  of  a  river  or  a  pond  higher  than  it  really  is  ;  we 
thus  consider  the  water  to  be  not  so  deep,  an  illusion  which  may 
be  dangerous. 


Fig.  252. 

The  same  cause  makes  a  stick  half  immersed  in  water  appear 
broken  when  it  is  looked  at  at  the  side  :  for  the  portion  out  of  the 
water  is  seen  in  its  true  position,  while  that  which  is  immersed 


326 


On  Light. 


[329- 


appears    raised,   from   which  results  the  appearance  of  the  stick 
being  broken  at  the  surface  of  the  liquid  (fig.  253). 

We  may,  in  conclusion, 
cite  the  influence  which 
refraction  exerts  upon  the 
apparent  rising  and  set- 
ting of  the  stars,  which  we 
can  see  a  little  before  they 
are  above  the  horizon,  and 
a  little  after  they  have 
sunk  below  it.  To  explain 
this  phenomenon  let  us 
suppose  the  atmosphere 
divided  into  layers  parallel 


Fig.  253. 


to  the  surface  of  the 
globe,  as  represented  in 
fig.  254.  Owing  to  the  pressure  exerted  by  the  upper  layers 
upon  the  lower  ones,  the  latter  are  more  dense  (129),  and  therefore 
more  refringent,  for,  as  we  have  seen,  the  refracting  power  of  the 


*  Fig.   254- 

air  increases  with  its  density  (327).  The  sun's  rays,  which  pene- 
trate the  atmosphere,  are  always  refracted  in  the  same  direction  as 
they  pass  from  one  layer  to  another  :  herice  their  path,  instead  of 
being  that  of  a  straight  line,  will  be  really  somewhat  curved.  Thus 
it  is  that,  while  the  sun  is  at  S,  below  the  horizon,  HH,  an  observer 


-330] 


Change  of  Refraction  to  Reflection, 


327 


at  A,  on  the  surface  of  the  earth,  will  see  it  raised  by  an  amount 
which  is  generally  equal  to  its  apparent  diameter. 

330.  Change  of  refraction  to  reflection. — Whenever  light 
passes  from  one  medium  into  a  more  refringent  one,  from  air  into 
water  for  instance,  there  is  nothing  to  prevent  the  refracted  ray 
from  approaching  the  normal,  to  form  an  angle  smaller  than  the 
angle  of  incidence  ;  but  if,  on  the  contrary,  the  second  medium  is 
less  refringent  than  the  first,  in  which  Case  the  refracted  fays  recede 
from  the  normal,  there 'is  a  limit  to  their  deviation,  and  hence  re- 
fraction may  become  impossible. 

To  get  a  clearer  idea  of  this,  let  us  imagine  a  hollow  glass 
sphere  half  filled  with  water  (fig.  255),  and  a  ray  of  light,  LA,  to 
enter  the  liquid  without  being  re- 
fracted, which  is  the  case  when  it 
penetrates  at  right  angles  the  small 
facette  which  we  can  always  con- 
ceive to  exist  at  the  point  at  which 
it  enters.  This  ray  is  refracted  at 
A  in  passing  from  water  into  air, 
and  diverges  from  the  normal, 
BAG,  in  the  direction  AR.  Now, 
conceive  the  luminous  body  to 
move  gradually  from  AC  ;  as  the 
angle  of  incidence,  CAL,  increases, 
the  angle  of  refraction,  BAR,  does 
so  too  ;  and  an  angle,  CAL,  might  acquire  such  a  magnitude  as  to 
emerge  parallel  to  the  surface,  AM,  of  the  liquid.  This  angle  ot 
incidence  is  what  corresponds  to  the  limit  of  refraction.  For,  for 
any  greater  angle  of  incidence,  the  angle  of  refraction  should  exceed 
the  angle  BAM  ;  and  the  light  would  then  take  below  AM  a  direc- 
tion such  as  Ar.  There  would,  however,  then  be  no  refraction,  for 
the  light,  always  travelling  through  water,  does  not  change  its 
medium.  If  the  incident  ray  be  then  represented  by  /A,  if  we 
measure  the  two  angles  /AC  and  CAr,  it  will  be  found  that  they 
are  exactly  equal,  which  shows  that  at  the  point,  A,  the  light  is 
reflected  according  to  the  ordinary  laws  of  reflection. 

This  kind  of  reflection  at  the  surface  which  separates  two  media 
of  different  refracting  power,  is  called  internal  reflection  :  it  is  also 
called  total  reflection,  for  here  the  whole  of  the  incident  light  is 
reflected,  which  is  never  the  case  in  ordinary  reflection,  even  in  the 
best  polished  surfaces  (309). 


Fig,  255. 


328 


On  Light. 


[330- 


The  phenomenon  is  frequently  met  with  ;  thus,  if  a  silver  spoon 
be  placed  in  a  glass  of  water,  and  it  be  raised  above  the  eye,  the 
surface  of  the  liquid  is  seen  brighter  than  a  polished  metal,  and 
one  portion  of  the  spoon  for  an  image  in  it  as  in  a  mirror.  Similar 
effects  are  met  with  in  aquariums.  The  upper  surface  of  the  liquid, 
when  looked  at  from  a  suitable  position  below,  gives  a  reflected 
image  of  the  object  it  contains. 

331.  Mirage. — The  mirage  is  an  optical- illusion  by  which  in- 
verted images  of  distant  objects  are  seen  as  if  below  the  ground,  or 
in  the  atmosphere.  This  phenomenon  is  of  most  frequent  occur- 
rence in  hot  climates,  and  more  especially  on  the  sandy  plains  of 


Fig.   256. 

Egypt.  The  ground  there  has  often  the  aspect  of  a  tranquil  lake, 
on  which  are  reflected  trees  and  the  surrounding  villages.  In  vain, 
however,  does  the  traveller  redouble  his  velocity  ;  this  imaginary 
lake  so  desired,  flees  from  him  as  he  advances.  The  phenomenon 
has  long  been  known  ;  but  Monge,  who  accompanied  Napoleon's 
expedition  to  Egypt,  was  the  first  to  give  an  explanation  of  it. 

It  is  a  phenomenon  of  refraction,  which  results  from  the  unequal 
density  of  the  different  layers  of  the  air  when  they  are  expanded  by 
contact  with  the  heated  soil.  The  least  dense  layers  are  then  the 
lowest,  and  a  luminous  ray  from  an  elevated  object,  a  (fig.  256), 
traverses  layers  which  are  gradually  less  refracting  ;  for,  as  we 
have  shown  (327),  the  refracting  power  of  a  gas  diminishes  wil'h 


-332]       Refraction  Through  Prisms  and  Lenses.        329 

lessened  density.  The  luminous  ray  continues  its  path,  being,  how- 
ever, more  and  more  bent  from  one  layer  to  the  other,  until  the  angle 
of  incidence,  which  continually  increases,  reaches  the  limit  at  which 
internal  reflection  succeeds  to  refraction  (330).  The  ray  then  rises 
at  a,  as  seen  in  the  figure,  and  undergoes  a  series  of  successive  re- 
fractions, but  in  a  direction  contrary  to  the  first,  for  it  now  passes 
through  layers,  which  are  gradually  more  and  more  dense,  and 
therefore  more  refracting.  The  luminous  ray  then  reaches  the  eye 
with  the  same  direction  as  if  it  had  proceeded  from  a  point  below 
the  ground,  and  hence  it  gives  an  inverted  image  of  the  object,  just 
as  if  it  had  been  reflected  at  the  surface  of  a  tranquil  lake. 

Mariners  sometimes  see  in  the  air  images  of  the  shores,  or  of 
distant  vessels.  These  are  called  fata  morgana.  This  is  due  to 
the  same  cause  as  the  mirage,  but  is  in  a  contrary  direction,  only 
occurring  when  the  temperature  of  the  air  is  above  that  of  the  sea, 
for  then  the  inferior  layers  of  the  atmosphere  are  denser,  owing  to 
their  contact  with  the  surface  of  the  water.  To  the  same  class  of 
phenomena  belong  the  tremulous  appearance  of  objects  seen  through 
the  current  of  hot  air  rising  from  a  chimney  or  a  spirit  lamp.  It  is 
related  that  in  this  way,  clandestine  stills  have  been  detected  by  the 
Revenue  officers. 


CHAPTER   IV. 

EFFECTS   OF   REFRACTION  THROUGH   PRISMS  AND   THROUGH 
LENSES. 

332.  Media  with  plane  parallel  faces. — When  a  luminous 
pencil  traverses  a  transparent  medium,  three  cases  may  be  con- 
sidered. First,  that  in  which  the  medium  is  comprised  between 
two  parallel  planes ;  second,  that  in  which  it  is  comprised  between 
two  plane  surfaces  inclined  towards  each  other  ;  thirdly,  that  in 
which  the  medium  is  comprised  between  two  curved  surfaces,  or 
between  a  curved  and  a  plane  surface,  which  gives  rise  to  the  same 
effects. 

We  will  start  with  the  consideration  of  the  first  case,  and  let  L;;z 
be  a  ray  of  light  traversing  a  glass  plate,  AB,  with  parallel  faces 
(fig.  257).  In  passing  from  air  into  glass  at  the  point  m,  this  ray 
approaches  the  normal ;  but  as,  on  its  emergence  from  the  glass  at 


330 


On  Light. 


[332- 


the  point  ;/,  it  deviates  from  the  perpendicular  by  exactly  the  same 
amount,  it  follows  that,  after  having  traversed  the  glass  plate,  its 

direction  On   is  exactly  pa-  111 1  ^     -  n  .    i "  v 

rallel    to    'Lm ;     whence  we  j,pjt?j 

conclude    that   light  is    not       ;!|j!JJ 

deviated  when  it  traverses  a 

medium  with  parallel  faces, 

such   as  the  glass  panes  in 

our  windows. 

333.    Prisms. — In  optics 
a  prism   is  any  transparent 


Fig.  258. 

medium  comprised  between  two  plane  faces  inclined  to  each  other. 
Thus,  the  facettes  of  a  glass  stopper  taken  in  pairs  form  as  many 
prisms. 

Fig.  258  represents  the  shape  and  arrangement  of  prisms  for 
optical  experiments.  It  is  a  piece  of  glass  cut  laterally  in  three 
plane  faces,  and  the  ends  of  which  are  also  equal  and  parallel 
triangular  faces.  The  three  right  lines  which  form  the  intersection 
of  two  faces  of  the  prism  are  called  the  edges.  The  mass  of  glass 
thus  cut  may  be  turned  about  an  axis  parallel  to  its  edges  ;  and  it 
is  moreover  mounted  on  a  stand  with  a  double  joint,  so  that  it  can 
be  placed  in  any  position  whatever. 

Prisms  produce  a  remarkable  effect  upon  light  which  traverses 
them.  First  a  deviation,  and  second  a  decomposition  into  various 
kinds  of  light.  Although  these  effects  are  always  simultaneous, 
we  shall  examine  the  first  by  itself :  the  second  will  be  afterwards 
investigated  under  the  head  of  dispersion. 


-334] 


Path  of  Rays  in  a  Prism. 


331 


334.  Path  of  rays  in  a  prism.— To  trace  the  path  of  a  ray  of 
light  in  passing  through  a  prism',  let  us  suppose  this  cut  by  a  plane 
perpendicular  to  its 
edges,  and  let  mno, 
(fig.  259)  be  the  sec- 
tion  thus  obtained.  If 
we  consider  the  path  of 
a  ray  of  light  La  along 
this  section  and  meet- 
ing the  prism  at  a, 
this  ray  approaches 
the  perpendicular  to 
the  surface  ;;/;/,  and 
takes  the  direction 
ab.  But  on  emerging 
from  the  prism  it  is  again  broken  in  the  same  direction,  being 


Fig.  259. 


Fig.  260. 

deflected  from  the  perpendicular  at  the  surface  mo  ;  for  it  passes 
into   a  less   refracting  medium.     It  forms   then   a    broken    line, 


332  On  Light.  [334- 


;  so  that  the  eye  which  receives  the  ray,  be,  which  is  called 
the  emergent  ray,  sees  the  object  in  the  direction  cbr  ;  that  is 
raised  towards  the  point  ;//  ;  which  is  expressed  by  saying  that  an 
object  seen  through  a  prism  appears  deflected  towards  the  summit  ; 
that  is,  the  edge  which  separates  the  faces  of  incidence  and  emer- 
gence. 

The  phenomenon  is  very  easily  demonstrated  by  observing 
through  a  prism  any  object  whatever,  as  represented  in  fig.  260. 
This  is  seen  to  be  raised  when  the  summit  of  the  prism  is  upper- 
most, and  lowered  when  the  summit  is  downward.  If  the  prism 
is  vertical,  the  image  is  displaced  either  to  the  right  or  to  the  left 
•  of  the  observer,  according  to  the  position  of  the  summit  in  either 
direction. 

This  property  which  prisms  have,  of  twice  deflecting  the  light  in 
the  same  direction,  is  the  basis  of  all  that  has  to  be  said  about 
lenses. 

LENSES. 

335.  Different  kinds  of  lenses.  —  In  optics  the  name  lens  is 
given  to  discs  of  glass  bounded  by  two  spherical  surfaces,  or  by  a 


Fig.  261.  tig.  262. 

plane  and  a  spherical  surface.  The  true  lens,  the  only  one  to 
which  the  name  is  stri'ctly  applicable,  is  that  in  which  both  surfaces 
are  bulged,  such  as  represented  in  a  side  view  in  fig.  261,  and  in 
front  in  fig.  262  ;  but  this  term  of  lens  has  been  extended  to  other 
masses  of  glass,  from  the  analogy  of  their  action  on  light. 


-336] 


Properties  of  Lenses. 


333 


They  are  usually  made  either  of  crown  glass,  which  is  free  from 
lead,  or  of  flint  glass,  which  contains  lead,  and  is  more  refractive 
than  crown  glass. 

The  combination  of  spherical  surfaces,  either  with  each  other  or 
with  plane  surfaces,  gives  rise  to  six  kinds  of  lenses,  sections  of 
which  are  represented  in  figs.  263,  264  ;  four  are  formed  by  two 
spherical  surfaces,  and  two  by  a  plane  and  a  spherical  surface. 

M  is  a  double  convex,  N  is  a  plano-convex,  O  is  a  converging 
concavo-convex ;  P  is  a  double  concave,  Q  is  a  plano-concave,  and 
R  is  a  diverging  concavo-convex.  The  lens  O  is  also  called  the 
converging  meniscus,  and  the  lens  R  the  diverging  meniscus. 


Fig.  263. 


Fig.   264. 


The  first  three,  which  are  thicker  at  the  centre  than  at  the 
borders,  are  converging  ;  the  others,  which  are  thinner  in  the 
centre,  are  diverging.  In  the  first  group,  the  double  convex  lens,  M, 
only  need  be  considered,  and  in  the  second  the  double  concave, 
P,  as  the  properties  of  each  of  these  lenses  apply  to  all  those  of 
the  same  group. 

336.  Principal  axis  ;  optical  centre  ;  secondary  axes. — Before 
describing  the  properties  of  double  convex  lenses,  we  must  premise 
some  definitions  analogous  to  those  already  given  for  mirrors.  We 
may  remark  that  a  double  convex  lens  is,  as  shown  in  fig.  265,  the 
portion  common  to  two  spheres,  which  intersect  each  other.  That 
being  premised,  the  centres  C  and  c  of  these  spheres  are  called  the 
centres  of  curvature  of  the  lens,  and  the  straight  line,  XY,  which 
passes  through  these  points,  is  the  principal  axis. 

Besides  these  two  centres  of  curvature,  there  is  a  remarkable 
point  in  the  lenses,  called  the  optical  centre.  This  name  is  given 
to  a  point  O,  on  the  principal  axes,  equidistant  from  the  two  faces 
of  the  lens  ;  at  all  events,  when  they  have  the  same  curvatures, 
which  is  the  usual  case.  Now  it  can  be  shown  by  geometrical 
considerations,  that  any  ray  of  light  which  passes  through  the 
optical  centre,  emerges  without  deflection ;  that  is,  it  comports 


334  On  Light.  [336- 

itself  just  as  if  it  traversed  a  medium  with  parallel  faces  (332), 
while  the  luminous  rays  which  do  not  pass  through  this  point  are 
deflected  twice  in  the  same  direction,  as  in  passing  through  prisms 
(334). 


<    K 


Fig.   265. 

Any  straight  line,  KH,  which  passes  through  the  optical  centre, 
without  passing  through  the  centres  of  curvature,  is  a  seconday  axis. 
There  is  only  one  principal  axis,  but  the  number  of  secondary  axes 
is  unlimited.  We  shall  subsequently  learn  that  the  principal  and 
the  secondary  axes  play  exactly  the  same  part  in  the  formation  of 
images  in  lenses,  as  they  do  in  concave  or  convex  mirrors. 

In  order  to  compare  the  path  of  a  luminous  ray  in  a  lens  with 
that  in  a  prism,  the  same  hypothesis  is  made  as  for  curved  mirrors 
(318),  that  is,  the  surfaces  of  these  lenses  are  supposed  to  be  formed 
of  an  infinity  of  small  plane  surfaces  or  elements  ;  the  normal  at 
any  point  is  then  the  perpendicular  to  the  plane  of  the  corresponding 
element  :  at  m,  for  instance,  it  is  the  straight  line  in  C  joining  the 
points  m  to  the  centre  of  curvature  ;  in  like  manner  at  n  the  normal 
is  en.  This  being  premised,  the  properties  of  lenses  are  easily  de- 
duced from  those  of  prisms  (334). 

337.  Path  of  rays  in  double  convex  lenses.  Foci. — The  rays 
of  light  which  traverse  a  lens  may  be  either  parallel  or  divergent; 
we  will  first  consider  the  former  case,  and  suppose  further  that  the 
rays  are  parallel  to  the  principal  axis,  as  shown  in  fig.  266.  Arguing 
on  the  above  hypothesis,  that  the  curved  surface  of  a  lens  is  an  as- 
semblage of  small  plane  facettes,  or  elements  inclined  towards  each 
other,  it  will  be  seen  that  the  ray  X,  which  coincides  with  the  prin- 
cipal axis,  traverses  the  lens  perpendicularly  to  the  facettes  on  en- 
trance and  emergence  ;  and  that,  therefore,  it  continues  to  travel  in 
a  right  line  as  traversing  in  reality  a  medium  with  parallel  faces. 
This,  however,  is  not  the  case  with  any  other  ray,  L,  more  or  less 


-338]  Conjugate  Focus.  335 

distant  from  the  principal  axes  ;  for  here  the  small  facettes  at  the 
points  of  incidence  and  emergence,  being  inclined  to  each  other  like 
the  faces  of  a  prism,  the  ray  is  twice  bent  in  the  same  direction,  so 
as  to  cut  the  principal  axis  in  a  point  F.  Any  other  ray,  M,  is  de- 
flected in  the  same  manner,  and  although  more  distant  from  the 
principal  axis,  will  cut  it  at  F  ;  which  arises  from  the  fact,  that  the 


Fig.  266. 

• 

two  opposite  facettes  at  the  points  of  entrance  and  emergence,  being 
the  more  inclined  to  one  another  the  nearer  they  are  to  the  edges 
of  the  lens,  impart  to  the  ray  a  greater  deviation.  All  rays  parallel 
to  the  axis  behave  in  the  same  manner  after  having  traversed  the 
lens,  and  it  can  thus  be  understood  how  a  parallel  pencil  is  trans- 
formed into  a  converging  pencil.  The  point  where  all  the  rays 
which  were  parallel  to  the  axis  coincide  is  called,  as  in  the  case  of 
mirrors,  the  principal  focus,  and  we  shall  represent  it  by  the  letter 
F.  It  may  be  formed  on  either  side  of  the  lens  according  to  the 
direction  in  which  light  is  propagated. 

The  position  of  the  principal  focus  is  fixed  and  is  easy  to  deter- 
mine ;  nothing  more  is  required  than  to  receive  on  the  lens  a  pencil 
of  parallel  rays,  a  pencil  of  solar  light  for  instance,  and  then  to  hold 
behind  the  lens  a  sheet  of  white  paper.  By  moving  this  a  position 
is  found  in  which  the  luminous  circle  formed  on  the  screen  attains 
its  maximum  lustre  :  this  point  is  the  principal  focus. 

338.  Conjugate  focus. — We  will  now  consider  the  case  in  which 
the  source  of  light  is  at  a  small  distance,  but  yet  further  than  the 
principal  focus  (fig.  267).  The  pencil  which  is  incident  upon  the 
lens  being  than  divergent,  it  follows  that,  after  having  traversed  the 
lens,  the  rays  converge  less  rapidly  than  in  fig.  266,  and  that,  there- 
fore, they  no  longer  coincide  in  F,  but  beyond,  in  a  point  /,  which 
is  called  the  conjugate  focus  of  the  point  L,  to  express,  as  in  con- 
cave mirrors,  the  correlation  of  these  two  points  ;  which  is  of  such 


336 


On  Light. 


[338- 


a  kind  that,  when  the  luminous  object  passes  from  L  to  /,  the  con- 
jugate focus  conversely  passes  from  /  to  L. 

The  position  of  the  conjugate  focus  is  not  fixed  ;  it  varies  with 
that  of  the  luminous  object :  the  nearer  this  is  to  the  lens  the  more 
distant  is  the  conjugate  focus,  as  shown  by  comparing  fig.  268  with 
fig.  267  ;  in  fact,  the  incident  rays  being  mqre  and  more  diverging 
the  emergent  rays  are  necessarily  so  too. 


Fig.  268. 

We  will  now  consider  the  case  in  which  the  luminous  object 
coming  continually  nearer  the  lens,  ultimately  coincides  with  the 
principal  focus  (fig.  269).  This  being  the  point  where  rays  parallel 
to  the  axis  coincide,  it  follows,  conversely,  that  luminous  rays, 
which  emanate  from  this  point,  pursue  in  the  opposite  direction  the 
same  path  as  in  arriving  ;  that  is  to  say,  that  they  form  on  emerging 
from  the  lens  a  pencil  parallel  to  the  axis,  and  that  in  the  present 
case  no  focus  can  be  produced  at  any  distance. 

339-  Virtual  focus. — We  have  still  to  consider  another  focus, 
the  virtual  focus.  Let  us  suppose  a  luminous  object  continually 
coming  nearer  the  lens,  ultimately  comes  between  it  and  the  prin- 
cipal focus  (fig.  270).  The  divergence  of  the  incident  pencil  being 
then  greater  than  in  fig.  269,  it  follows  that  the  rays  after  emergence 
will  be  more  and  more  spread  out  than  in  this  figure :  they  should, 
therefore,  become  divergent,  as  shown  in  the  pencil  MN.  The  eye 
which  receives  these  rays  will  suppose  that  they  proceed  from  the 
point  /,  where  their  prolongations  coincide.  In  this  point  the 


-340]          Properties  of  Double  Convex  Lenses. 


337 


luminous  object  will  appear  ;  it  is  not  however  a  virtual  focus,  but 
only  an  optical  illusion,  just  like  that  in  a  concave  mirror,  when  the 
luminous  object  is  placed  between  the  mirror  and  its  principal 
focus. 

340.  Summary  of  properties  of  double  convex  lenses. — 
From  what  has  been  said,  we  may  deduce  the  three  following  prin- 
ciples as  to  the  properties  of  double  convex  lenses. 

I.  Luminous  rays  parallel  to  the  axis,  after  having  traversed  a 
double  convex  lens,  coincide  in  a  single  point  which  is  the  princi- 
pal focus  (fig.  266)  ;  and  conversely,  rays  from  this  focus  form,  on 
their  emergence  from  the  lens,  a  pencil  parallel  to  the  axis  (fig.  269). 

Fig.  269. 


Fig.  270. 

II.  Luminous  rays  emitted  from  a  point  outside  the  principal 
focus  emerge  convergent  from  the  lens,  and  coincide  in  a  point 
called  the  conjugate  focus  (fig.  267),  which  is  formed  at  a  greater 
distance  behind  the  lens,  the  nearer  is  the  luminous  object  to  the 
principal  focus  (fig.  268). 

I 1 1.  Finally,  the  rays  from  a  point  between  the  lens  and  the 
principal  focus  emerge  divergent,  and  give  rise  to  a  virtual  focus  on 
the  same  side  as  the  object  (fig.  270). 

These  ideas  in  reference  to  foci  are  indispensable  in  the  expla- 
nation of  the  formation  of  images  by  lenses. 


338 


On  Light. 


[341- 


FORMATION   OF   IMAGES   IN    LENSES. 

341.  Real  images  in  double  convex  lenses. — The  refraction 
of  light  in  double  convex  lenses  gives  rise  to  images,  which  are 
quite  comparable  to  those  seen  by  reflection  in  concave  mirrors 
(322),  and  which  like  these  are  of  two  kinds  raz/and  virtual, 

We  will  first  consider  the  case  of  the  real  image.  This  is 
formed  whenever  any  object  is  placed  in  front  of  a  condensing 
lens  outside  its  principal  focus  ;  the  lens  reproduces  then  on  the 
other  side  a  reversed  image  of  the  object,  which,  may  be.  caught 
upon  a  screen  (fig.  271),  and  is  not  less  remarkable  for  the  fidelity 
of  the  colour  than  for  the1  accuracy  of  the  outline  ;  this  is  the  real 
image.  Its  formation  may  be  readily  understood  by -reference  to 


Fig.  271. 

what  has  been  said  about  conjugate  foci  (338).  Yet  it  must  be 
added,  that  as  all  the  properties  of  the  principal  axis  apply  also 
to  the  secondary  axes,  it  follows  that  as  a  point  on  the  principal 
axis  has  always  its  focus  on  this  axis,  so  also  any  point  on  the 
secondary  axis  has  its  focus  oh  the  latter.  Hence,  in  the  above 
figure,  all  rays  from  the  point  A  converge  at  a  on  the  secondary 
axis  through  this  point,  and  form  the  conjugate  focus  of  this  point, 
that  is  to  say,  its  image.  In  like  manner,  the  image  of  the  point 
B  forms  at  b,  and  as  the  same  is  the  case  for  all  points  of  the 
object,  the  result  is  a  series  of  conjugate  foci ;  these  in  their 
entirety  constitute  the  image  ab,  which  is  inverted  and  smaller : 
the  reversal  arises  from  the  crossing  of  the  secondary  axes  between 


-341]        Real  Images  in  Double  Convex  Lenses.  339 

the  object  and  the  image,   and  its  smallness  from  its  being  formed 
nearer  the  lens  than  the  object  is. 

Yet  the  image  is  not  always  smaller  than  the  object  ;  it  may  be 
greater.  For,  from  the  reciprocity  of  position  between  the  object 
and  its  conjugate  focus  (340),  if,  in  fig.  271,  ab  were  the  object, 
then  as  the  luminous  rays  pursue  the  same  path,  but  in  the  opposite 
direction,  the  image  would  be  formed  at  AB,  reversed  as  before, 
but  larger.  A  double  convex  lens  may  thus  give  real  images, 


Fig.  272. 

which  are  either  smaller  or  larger  than  the  object.  This  may  be 
verified  by  the  following  experiment:  in  a  dark  room  a  double 
convex  lens  is  placed,  and  in  front  of  it,  but  some  yards  beyond 
the  principal  focus,  a  lighted  candle.  If  then  there  is  placed 
behind  the  lens  a  screen,  which  can  be  placed  more  or  less  near, 
a  position  is  found  in  which  there  is  produced  on  the  screen  a  very 
small  and  inverted  image  of  the  candle,  as  shown  in  fig.  272.  If, 
on  the  contrary,  the  lens  be  brought  nearer  the  candle,  and  at  the 

z  2 


340  On  Light.  [341- 

same  time  the  distance  of  the  screen  be  increased,  a   reversed 
image  is  obtained,  but  it  is  greatly  enlarged  (fig.  273). 

This  principle,  that  double  convex  lenses  'give  real  and  very 
small  images  of  distant  objects,  and,  on  the  contrary,  greatly 
magnified  images  of  near  objects,  will  meet  with  numerous  applica- 
tions in  the  optical  instruments,  which  will  be  presently  described, 
such  as  the  microscope,  the  telescope,  magic  lantern,  phantasma- 
goria. 


Fig.  273. 

342.  Virtual  images  in  double  convex  lenses. — Besides  the 
real  images  we  have  just  considered,  double  convex  lenses  give 
also  virtual  images,  which  are  produced  under  the  same  conditions 
as  the  virtual  foci  ;  that  is,  when  the  object  is  between  the  lens  and 
the  principal  focus.  For  let  an  object,  ab,  be  placed  between  a 
double  convex  lens  and  its  principal  focus  :  applying  here  what  was 
previously  said  in  reference  to  virtual  foci,  we  know  that  all  rays 
proceeding  from  any  point,  «,  of  the  object  emerge  while  diverging, 
and  reach  the  eye  as  if  they  proceeded  from  the  point  A,  where 


-343]  Double  Concave  Lenses.  341 

the  prolongation  of  the  same  rays  coincide,  and  where  there  is 
formed  for  the  eye  the  virtual  image  of  the  point  a.  For  the  same 
reason  the  eye  sees  at  B  the  image  of  b ;  hence  the  image  of  AB 
appears  at  ab,  but  it  is  virtual ;  that  is  to  say,  it  does  not  really 
exist,  it  could  not  be  received  on  a  screen,  and  is  only  an  optical 
illusion . 


Fig.  274. 

It  is  to  be  remarked  that  in  opposition  to  what  takes  place  when 
the  image  is  real,  the  virtual  image  is  erect,  and  in  all  cases  larger 
than  the  object  ;  the  rectification  of  the  image  arises  from  the  fact 
that  the  secondary  axes  do  not  intersect  between  the  image  and 
the  object,  but  beyond  it  ;  the  magnification  arises  from  the  image 
being  further-than  the  object  from  the  point  of  intersection  of  the 
secondary  axes  which  pass  through  a  and  b. 

The  term  lens  is  applied  to  the  lenticular  glasses  used  as  mag- 
nifying glasses.  Everyone  is  aware,  that  if  the  print  of  a  book  be 
closely  looked  at  through  such  a  lens  it  will  appear  larger  ;  if 
fhe  lens  be  progressively  removed,  a  moment  is  reached  when  the 
characters  disappear.  This  is  the  case  when  they  are  in  the  prin- 
cipal focus  :  when  it  is  still  further  removed  the  characters  re- 
appear ;  but  they  are  reversed  for  then  they  are  beyond  the  prin- 
cipal focus. 

343.  Double  concave  lenses  ;  foci  and  images. — We  have  seen 
in  speaking  about  double  convex  lenses,  that  as  the  thickness  de- 
creases from  the  centre  towards  the  edges,  the  small  plane  facettes, 
corresponding  to  the  incidence  and  convergence  of  the  same  ray, 
are  more  and  more  inclined  from  the  centre  to  the  periphery. 


342  On  Light.  [343- 

But  in  double  concave  lenses,  on  the  contrary,  where  the  thickness 
increases  from  the  centre  to  the  edge,  the  small  facettes  are  more 
and  more  apart ;  and  hence  the  opposite  phenomena.  For,  while 
double  convex  lenses  cause  the  rays  traversing  them  to  coincide, 
by  breaking  them  twice  in  the  same  direction,  so  as  to  bring  them 
nearer  the  principal  axis,  double  concave  lenses  produce  the  oppo- 
site effect,  and  only  increase  the  divergence  of  the  rays. 

This  phenomenon  may  be  readily  understood  by  reference  to 
fig.  275,  in  which  it  is  apparent  how  the  rays  are  twice  broken  in 
the  same  direction,  so  as  to  diverge  from  the  axis,  and  give  rise  to 
the  diverging  pencil  MN.  But  the  eye  which  receives  this  pencil 
is  acted  upon  by  it,  as  if  the  luminous  object  were  at  /;  there  is 
thus  produced  a  virtual  fccus,  the  only  one  possible  in  double  con- 
cave lenses. 


As  these  kinds  of  lenses  '  ave  only  virtual  fcci,  they  can  produce 
none  but  virtual  images  ;  these  images  are  moreover  always  erect 
and  smaller  than  the  object.  Thus  let  AB  be  an  object  seen 
through  a  double  concave  lens  (fig.  276)  ;  the  luminous  pencil 
from  A  is  deflected  on  passing  through  the  lens  in  such  a  manner 
as  to  reach  the  eye  as  if  it  were  emitted  from  a  point,  a,  on  the 
secondary  axis,  AO.  In  like  manner,  the  pencil  from  the  point, 
B,  reaches  the  eye  as  if  it  started  from  the  point,  b.  There  is  formed 
therefore  at  ab,  between  the  secondary  axes,  AO  and  BO,  a  virtual 
image  of  the  object,  AB,  which  is  smaller  and  erect.  This  image 
is  necessarily  always  smaller  than  the  object,  for  it  is  nearer  the 
point,  O,  where  the  secondary  axes  intersect. 

APPLICATION   OF   LENSES. 

344.  Refraction  of  beat. — When  a  pencil  of  the  sun's  rays  is 
received  on  a  condensing  lens,  not  merely  is  light  concentrated  on 


-344] 


Refractiort  of  Heat. 


343 


its  focus,  but  heat  also  ;  for  if  a  piece  of  an  inflammable  substance, 
such  as  amadou,  paper,  cloth,  wood,  be  placed  in  the  focus,  the 
body  soon  begins  to  burn.  With  lenses  of  large  diameter  metals 
even  may  be  melted. 

This  property  which  condensing  lenses  have  is  utilised  for  pro- 
ducing fire  in  what  are  called  burning  glasses.  They  may  be  a 
source  of  danger,  by  becoming  a  source  of  fire,  when  a  lens  is  ex- 
posed to  the  solar  rays.  The  same  accident  may  be  produced  by 
spherical  glass  vessels  filled  with  water  ;  for  they  refract  the  light 
and  heat  like  double  convex  lenses.  A  drop  of  water,  too,  on  a 
leaf,  concentrates  the  sun's  rays,  and  frequently  marks  the  leaf. 


Fig.  276. 

The  concentration  ot  the  heat  rays  of  the  sun  has  received  an 
application  in  certain  sun  dials,  when  the  hour  of  midday  is 
marked  by  the  discharge  of  a  small  cannon  (fig.  277).  Above  the 
cannon  is  a  condensing  lens,  the  focus  of  which  exactly  corresponds 
to  the  touch-hole  of  the  cannon  the  moment  the  sun  passes  the 
meridian  of  this  place.  Hence,  the  cannon  being  charged  and 
primed  beforehand,  the  lens  ignites  the  powder  just  at  midday,  and 
the  explosion  announces  the  time  at  a  distance. 

Yet  the  time  thus  given  is  what  is  called  in  astronomy  solar 
time,  or  true  time,  in  which  the  length  of  day  varies.  Now  our 
watches  and  clocks  being  regulated  for  mean  time,  that  is  to  say, 


344 


On  Light. 


[344- 


for  an  unchangeable  day,  only  agree  with  the  sun  four  times  a 
year;    December  24,  April    15,  June   15,  and  September   i.      On 


Fig.  277. 

February  11  a  clock  giving  mean  time  is  14'  37"  faster  than  the 
sun,  and  on  November  3  it  is  16'  17"  slow.  The  equation  of  titjic 
represents  the  amount  which  on  all  the  days  of  the  year  must  be 
added  to  or  taken  from  the  time  of  a  clock  to  obtain  the  mean 
time.  Hence,  strictly  speaking,  it  is  incorrect  tg  use  the  ordinary 
expression,  that  a  good  watch  or  a  good  clock  goes  like  the  sun. 

345.  Beacons.  Lighthouses. — These  are  fires  lighted  at  night 
on  high  towers  along  the  shores  of  the  sea,  in  order  to  guide 
mariners  in  darkness  and  enable  them  to  keep  clear  of  danger. 

Beacon  fires  were  originally  wood  or  coal  fires  ;  but  these  were 
dull  and  unsteady.  They  were  afterwards  replaced  by  oil  lamps 
placed  in  the  principal  focus  of  concave  reflectors,  which  sent  the 
reflected  light  to  a  great  distance,  for  its  rays  were  parallel. 

In  1822  Fresnel  made  a  great  improvement  in  the  illumination 
of  lighthouses  as  they  are  now  called.  Abandoning  the  use  ot 
metallic  reflectors,  which  soon  tarnished  under  the  influence  of  the 
sea-fogs,  Fresnel  substituted  large  plano-convex  lenses,  in  the 
focus  of  which  he  placed  a  powerful  lamp  with  four  concentric 
wicks,  and  equal  in  illuminating  power  and  quantity  of  oil  con- 
sumed to  seventeen  Carcel  lamps.  But  the  difficulty  of  construct- 
ing such  lenses,  which  must  necessarily  be  large,  and  at  the  same 


-345] 


Lighthouse  Lenses. 


345 


time  not  thick,  so  as  not  to  absorb  much  light,  led  Fresnel  to 
adopt  a  special  system  of  lenses,  known  as  echelon  or  lighthouse 
lenses. 

Seen  in  front  in  fig.  278,  and  in  profile  in  fig.  279,  they  consist 
of  a  plano-convex  lens,  A,  a  foot  in  diameter,  round  which  are 
arranged  eight  or  ten  glass  rings,  which  are  also  plano-convex,  and 
whose  curvature  is  calculated,  so  that  each  has  the  same  focus  as 
the  central  lens,  A.  A  lamp  being  placed  in  the  focus  of  this 
refracting  system,  an  immense  horizontal  pencil,  RC,  is  formed, 
which  sends  the  light  to  a  great  distance^.  Further,  above  and 
below  these  lenses,  are  placed  several  silvered  glass  mirrors,  mn. 


Fig.  279. 

Thus  the  rays,  which  would  be  lost  towards  the  sky  and  the  earth, 
are  utilised  and  sent  in  a  horizontal  direction.  By  this  double 
combination  a  vast  horizontal  pencil  is  obtained,  which  sends  the 
light  of  the  lamp  to  a  distance  of  20  or  30  miles  ;  but  it  only  sends 
it  in  one  direction.  To  increase  the  number  of  points  of  the 
horizon  at  which  the  light  can  be  seen,  Fresnel,  instead  of  a  single 
system  of  lenses  and  mirrors  represented  in  fig.  279,  united  eight 
such  arrangements,  so  as  to  form  an  enormous  glass  pyramid  with 
eight  faces,  as  seen  in  fig.  280,  which  represents  a  lighthouse  lens 
of  the  largest  size,  constructed  by  M.  Soutter,  and  exhibited  at  the 


346 


On  Light. 


[345- 


Paris  Universal  Exhibition  in  1855.      The  system  of  mirrors  and 
lenses  alone  is  10  feet  high. 

A  lighthouse  lens  of  this  kind  sends  a  powerful  beam  of  light 
towards  eight  points  of  the  horizon,  but  all  other  points  are  desti- 
tute of  light,  so  that  vessels  sailing  in  these  dark  parts  would  have 


Fig.  280. 

no  help  from  the  lighthouse.  This  difficulty  was  removed  by  Fresnel 
by  means  of  a  very  simple  mechanism,  represented  at  the  lower  part 
of  fig.  280.  A  clockwork  motion,  M,  moved  by  a  weight,  P,  im- 
parts to  the  whole  system  of  lenses,  AB,  a  slow  rotating  motion  on 
six  rollers.  During  a  complete  revolution  of  the  apparatus,  the 


-345]  Lighthouse  Lenses.  347 

whole  horizon  is  successively  illuminated,  and  the  mariner  lost  in 
the  night,  sees  the  light  alternately  appear  and  disappear  after 
equal  intervals  of  time.  These  alternations  serve  to  distinguish 
lighthouses  from  an  accidental  fire  or  a  star.  By  means  too  of  the 
number  of  times  the  light  disappears  in  a  given  time,  and  by  the 


Fig.  281. 

colour  of  the  light,  sailors  are  enabled  to  distinguish  the  lighthouses 
from  one  another,  and  hence  to  know  their  position. 

Of  late  years  the  use  of  the  electric  light  has  been  substituted 
for  that  of  oil  lamps  ;  a  description  of  the  apparatus  will  be  given 
in  a  subsequent  chapter. 


348 


On  Light. 


[346- 


CHAPTER   V. 

DECOMPOSITION   OF   LIGHT   BY   PRISMS. 

346.  Solar  spectrum. — In  speaking  of  prisms  and  lenses,  we 
have  only  considered  the  change  in  direction  which  these  transpa- 
rent media  impart  to  luminous  rays,  and  the  images  which  result 
therefrom  ;  but  the  phenomenon  of  refraction  is  by  no  means  so 
simple  as  we  have  hitherto  assumed  :  when  white  light,  or  that 
which  reaches  us  from  the  sun,  passes  from  one  medium  into 
another,  //  is  decomposed  into  several  kinds  of  lights,  a  phenomenon 
to  which  the  name  dispersion  is  given. 


rig.  282. 

In  order  to  show  that  white  light  is  decomposed  by  refraction,  a 
pencil  of  solar  light,  SA  (fig.  282),  is  allowed  to  pass  through  a  small 


-347]  TJie  Colours  of  the  Spectrum.  349 

aperture  in  the  window  shutter  of  a  dark  chamber.  This  pencil 
tends  to  form  a  round  and  colourless  image  of  the  sun  on  the  screen ; 
but  if  a  flint  glass  prism  arranged  horizontally  be  interposed  in  its 
passage,  the  beam,  on  emerging  from  the  prism,  becomes  refracted 
towards  its  base,  and  produces  on  a  distant  screen  a  vertical  band, 
coloured  in  all  the  tints  of  the  rainbow,  which  is  called  the  solar 
spectrum,  rounded  at  the  ends.  In  this  spectrum,  the  production  of 
which  forms  one  of  the  most  brilliant  optical  experiments,  there  is, 
in  reality,  an  infinity  of  different  tints,  which  imperceptibly  merge 
into  each  other  ;  but  with  Newton,  it  is  customary  to  distinguish 
seven  principal  colours,  as  seen  in  the  coloured  plate.  These  are 
violet,  indigo,  blue,  green,  yellow,  orange,  red :  they  are  arranged 
in  this  order  in  the  spectrum,  the  violet  being  the  most  refrangi- 
ble, and  the  red  the  least  so.  They  do  not  all  occupy  an  equal 
extent  in  the  spectrum,  violet  having  the  greatest  extent,  and  orange 
the  least. 

From  the  experiment  of  the  solar  spectrum  Newton  concluded 
that  white  light,  that  is,  light  coming  from  the  sun,  is  not  homo- 
geneous, that  is  simple  ;  but  consists  of  seven  different  lights  which, 
united,  give  the  impression  of  white,  while,  when  separated, 
each  produces  its  own  colour.  He  ascribed  the  separation  of  these 
seven  lights  on  their  passage  through  the  prism  to  their  different 
degrees  of  refrangibility.  For  if  they  were  all  equally  refrangible, 
as  they  would  be  equally  bent  on  entering  and  emerging1  from  the 
prism,  they  would  traverse  it  without  being  separated,  and  the  light 
would  be  wThite  on  emerging  as  well  as  on  incidence. 

347.  The  colours  of  the  spectrum  are  simple. — If  one  of  the 
colours  of  the  spectrum  (the  yellow,  for  instance)  be  isolated  by  in- 
tercepting the  others  by  means  of  a  screen,  and  if  the  light  thus 
intercepted  be  allowed  to  pass  through  a  second  prism,  it  is  de- 
flected, but  without  decomposition  ;  that  is,  it  only  gives  rise  to  a 
single  emergent  pencil.  As  the  same  phenomenon  is  observed 
with  the  other  colours  of  the  spectrum,  it  is  concluded  that  they 
are  indecomposable  by  the  prism,  which  is  expressed  by  saying 
that  the  seven  colours  of  the  spectrum  are  simple  ox  primitive  colours. 

As  regards  the  cause,  in  virtue  of  which  one  part  of  the  spectrum 
produces  on  us  the  sensation  of  red,  another  of  yellow,  another  of 
orange,  and.so  forth,  the,  undulatory  theory  teaches  us  that  it  depends 
upon  the  number  of  vibrations  performed  by  the  molecules  of  ether. 
This  number,  which  is  very  great,  differs  with  each  colour,  and 
increases  from  red  to  violet.  Fresnel  has  calculated  that  for  the 


350  On  Light.  [347- 

extreme  red  it  is  458  millions  of  millions  in  a  second,  and  for  violet 
727  millions  of  millions.  As  the  velocity  of  propagation  is  the  same 
for  all  the  colours  of  the  spectrum,  but  each  corresponds  to  an  un- 
equal number  of  vibrations,  it  follows  that  the  length  of  these 
vibrations  must  vary  with  different  colours.  It  has  been  calcu- 
lated that,  in  the  case  of  red,  the  length  of  the  vibration  is  620 
millionths  of  a  millimetre,  and  for  violet  42  5  millionths. 

348.  Xiiiminous,  calorific,  and  chemical  effects    of  the  spec- 
trum.— The  various  spectral  rays  differ  not  only  in  their  colour, 
but  also  in  their  luminous  power,  in  the  heat  by  which  they  are 
accompanied,  and  by  the  chemical  effects  to  which  they  give  rise. 
It  is  found  that  the  middle  pencils,  the  yellow  and  the  green  illu- 
minate the  most  powerfully.     Thus  the  print  of  a  book  placed  in 
the  yellow  pencil  is  seen  more  distinctly  than  in  the  red  or  violet. 

The  calorific  action  of  the  spectrum  is  demonstrated  by  succes- 
sively placing  a  very  delicate  thermometer  in  the  various  parts  of 
the  spectrum.  e  It  is  observed  that  in  the  red,  or  even  a  little  beyond 
it,  the  heat  attains  its  greatest  intensity.  This  proves  the  existence 
of  invisible  heat  rays,  which  are  less  refrangible  than  all  other 
spectral  rays  (216). 

Passing  from  the  calorific  action  of  light  to  its  chemical  action, 
we  may  first  observe  that  it  tends  to  destroy  most  vegetable  colours, 
such  as  wall  papers  and  dyed  stuffs,  which  rapidly  fade  if  exposed 
to  bright  light.  Some  chemical  substances  are  known  which  are 
naturally  white,  and  are  blackened  by  the  luminous  rays  :  there  are 
gaseous  mixtures,  also,  which  suddenly  explode  when  exposed  to 
the  sun's  rays.  These  chemical  effects  are  not  produced  equally 
in  all  the  parts  of  the  spectrum  ;  the  greatest  chemical  action  is  met 
with  in  the  violet,  and  even  a  little  beyond. 

We  may  thus  say  that  the  heating  effect  is  met  with  in  the  ex- 
treme red,  the  luminous  in  the  yellow,  and  the  chemical  action  in 
the  extreme  violet. 

349.  Dark  lines  of  the  spectrum. — The  colours  of  the  solar 
spectrum  are  not  perfectly  continuous  :  throughout  the  whole  ex- 
tent of  the  spectrum  are  a  great  number  of  very  narrow  dark  lines. 
They  are  best  observed  by  admitting  a  pencil  of  solar  rays  into  a 
darkened  room  through  a  narrow   slit.     If  at  a  distance  of  three  or 
four  yards  we  look  at  this  slit  through  a  flint  glass  prism,  with  its 
edge  held  parallel  to  the  edge  of  the  slit,  we  observe  a  number  of 
very  delicate  dark  lines  parallel  to  the  edge  of  the  prism,  and  at 
very  unequal  intervals. 


-351]  Spectroscope.  351 

The  existence  of  these  dark  lines  was  first  observed  by  Wollas- 
ton  in  1802  ;  but,  Fraunhofer,  a  celebrated  optician  of  Munich,  first 
studied  and  gave  a  detailed  description  of  them.  He  mapped  the 
lines,  and  denoted  the  most  marked  of  them  by  the  letters  A,  a  B,  C 
D,  E,  b,  F,  G,  H  ;  they  are  therefore  generally  known  as  Fraun- 
hofeijs  lines. 

The  dark  line  A  (see  fig.  i  of  the  coloured  plate)  is  towards  the 
end,  and  B  in  the  middle  of  the  red  ray  ;  C  is  in  the  red  but  rather 
nearer  the  orange  ray  ;  D  is  in  the  orange  ray,  E  in  the  yellow,  F 
in  the  transition  from  green  to  blue,  G  in  the  indigo,  H  in  the 
violet.  There  are  certain  other  noticeable  dark  lines,  such  as  a  in 
the  red,  and  b  in  the  green.  In  the  case  of  the  sun's  light  the  posi- 
tions of  the  dark  lines  are  fixed  and  definite  ;  in  the  spectra  of  arti- 
ficial lights  and  of  the  fixed  stars  the  relative  positions  of  the  dark 
lines  vary.  For  the  electric  light  there  are  bright  lines  instead  of 
dark  ones ;  and  in  coloured  flames,  that  is  to  say  flames  in  which 
certain  chemical  substances  are  being  evaporated,  the  dark  lines  are 
replaced  by  very  brilliant  lines  of  light,  which  differ  with  different 
substances. 

350.  Spectrum  analysis. — This  property  of  coloured  flames  was 
first  discovered  by  Sir  John  Herschell,  who  remarked  that  by  vola- 
tilising substances  in  a  flame  a  very  delicate  means  is   afforded  of 
detecting  certain  ingredients  by  the  colours  they  impart  to  certain 
parts    of    the   spectrum  ;    and    Fox    Talbot,   in     1834,    suggested 
optical  analysis  as  probably  the  most  delicate  means  of  detecting 
minute  portions  of  a  substance.     To  Kirchhoff  and  Bunsen,  how- 
ever is  really  due  a  method  of  basing  on  the  observation  of  these 
lines  a  method  of  analysis.     They  ascertain  that  salts  of  the  same 
metal,  when  introduced  into  a  flame,  always  produce  lines  which 
are  identical  in  colour  and  position,  but  different  in  colour,  posi- 
tion, or  number,  for  different  metals  ;  and,  finally,  that  an  exceed- 
ingly small  quantity  of  a  metal  suffices  to  disclose  it's  existence.    Hence 
has  arisen  a  new  method  of  analysis  known  as  spectrum  analysis. 

351.  Spectroscope. — The  name  spectroscope  has  been  given  to 
the  apparatus  used  by  Kirchhoff  and  Bunsen  for  the  study  of  the 
spectrum.     One  of  the  forms  of  this  apparatus  is  represented  in  fig. 
283.     It  consists  of  three  telescopes  mounted  on  a  common  foot, 
and  whose  axes  converge  towards  a  prism,  P,  of  flint  glass.     The 
telescope  A  is  the  one  through  which  the  spectrum  is  observed  ;  it 
is  focussed  by  means  of  the  milled  head-screw  m.     The  telescope 
B  has  a  slit  the  width  of  which  can  be  regulated  by  the  screw  v. 

, 


352 


On  Light. 


[351- 


k  is  what  is  known  as  Bunsetfs  burner,  in  which  coal  gas  is 
burned,  mixed  with  air  in  such  a  manner  that  a  flame  of  little  or  no 
luminosity,  but  of  intense  heat,  is  produced.  The  substance  to  be 
examined  is  placed  in  this,  either  in  the  solid  form,  or  in  a  state  of 
solution  on  the  platinum  wire  at  the  end  of  the  support,  c.  It  is  thus 
volatilised  by  the  intense  heat,  and  the  flame  G  is  coloured.  The 
rays  emitted  from  this  flame  pass  through  the  slit  and  through  a 
system  of  lenses,  so  that  on  emerging  they  form  a  parallel  pencil  of 
rays  which  falls  on  the  prism  P.  Here  they  are  refracted  and  de- 


Fig.  283. 

composed  and  form  the  prismatic  spectrum.  The  spectator,  on 
looking  through  the  telescope  A,  sees  a  real  and  inverted  image  of 
the  spectrum. 

The  telescope  C  has  a  different  function  ;  it  contains  a  micro- 
metric  scale  photographed  on  glass,  so  that  it  is  white  on  a  dark 
ground.  The  light  from  the  candle  passing  through  the  scale  and 
the  lens  in  C  falls  in  parallel  rays  on  the  face  of  the  prism  P,  ard  is 
reflected  from  thence  through  the  object-glass  of  A,  so  that  the  ob- 
server seeing  the  spectrum  and  the  scale  simultaneously  can  exactly 


-352]  Experiments  with  the  Spectroscope.  353 

measure  the  relative  distances  of  the  various  spectral  lines.  M  is 
a  metal  cap  with  three  apertures  which  covers  the  prism  so  as  to 
exclude  the  diffused  light. 

352.  Experiments  with  the  spectroscope. — The  coloured 
plate  shows  certain  spectra  observed  by  means  of  the  spectroscope. 
Fig.  I.  represents  the  solar  spectrum. 

Fig.  II.  shows  the  spectrum  of  potassium.  It  is  continuous  ; 
that  is,  it  contains  all  the  colours  of  the  solar  spectrum  ;  moreover, 
it  is  marked  by  two  bright  lines,  one  in  the  extreme  red,  corres- 
ponding to  Fraunhofer's  dark  line  A  ;  the  other  in  the  extreme  violet. 

Fig.  III.  shows  the  spectrum  of  sodium.  This  spectrum  con- 
tains neither  red,  orange,  green,  blue,  nor  violet.  It  is  marked  by  a 
very  brilliant  yellow  ray  in  exactly  the  same  position  as  Fraunhofer's 
dark  line  D.  Of  all  metals  sodium  is  that  which  possesses  the 
greatest  spectral  sensibility.  In  fact,  it  has  been  ascertained  that 
ohe  two  hundred  millionth  of  a  grain  of  common  salt  is  enough  to 
cause  the  appearance  of  the  yellow  line  of  sodium.  Consequently 
it  is  very  difficult  to  avoid  the  appearance  of  this  line.  A  very 
little  dust  scattered  in  the  apartment  is  enough  to  produce  it,  which 
shows  how  abundantly  sodium  is  diffused  throughout  nature. 

Figs.  IV.  and  V.  show  the  spectra  of  cczsium  and  rubidium, 
metals  discovered  by  Bunsen  and  Kirchhoff  by  means  of  spectral 
analysis.  The  former  is  distinguished  by  two  blue  lines,  the  latter 
by  two  very  brilliant  red  lines  and  by  two  less  intense  violet  lines. 
A  third  metal,  thallium,  has  been  discovered  by  the  same  method 
by  Mr.  Crookes  in  England,  and  independently  by  M.  Lamy  in 
France.  Thallium  is  characterised  by  a  single  green  line. 

Still  more  recently  Richter  and  Reich  have  discovered  a  new 
metal  associated  with  zinc,  and  which  they  call  indium,  from  a 
couple  of  characteristic  lines  which  it  forms  in  the  indigo. 

The  extreme  delicacy  of  the  spectrum  reactions,  and  the  ease 
with  which  they  are  produced,  constitute  them  a  most  valuable 
help  in  the  qualitative  analysis  of  the  alkalies  and  alkaline  earths. 
It  is  sufficient  to  place  a  small  portion  of  the  substance  under  exam- 
ination on  platinum  wire  as  represented  in  fig.  283,  and  compare 
the  spectrum  thus  obtained  either  directly  with  that  of  another  sub- 
stance, or  with  the  charts  in  which  the  positions  of  the  lines  pro- 
duced by  the  various  metals  are  laid  down. 

With  other  metals  the  production  of  their  spectrum  is  more 
difficult,  especially  in  the  case  of  some  of  their  compounds.  The 
heat  of  a  Bunsen's  burner  is  insufficient  to  vaporise  the  metals,  and 

A  A 


354 


On  Light. 


[352- 


a  more  intense  temperature  must  be  used.  This  is  effected  by 
taking  electric  sparks  between  wires  consisting  of  the  metal  whose 
spectrum  is  required,  and  the  electric  sparks  are  most  conveniently 
obtained  by  means  of  RuhmkorfFs  coil.  Thus  all  the  metals  may 
be  brought  within  the  sphere  of  spectrum  observations. 


Fig.  284. 

353.  Decomposition  of  white  light. — Not  merely  can  white 
light  be  resolved  into  lights  of  various  colours,  but  by  combining 
the  different  pencils  separated  by  the  prism,  white  .  light  can  be  re- 
produced. This  may  be  effected  in  various  ways  : 

I.  A  pencil  of  solar  light  is  decomposed  by  a  prism,  as  shown 
in  fig.  284,  and  the  spectrum  is  caught,  not  on  a  screen,  but  on  a 
rather  large  double  convex  lens,  in  the  focus  of  which  is  placed  a 
small  cardboard  or  ground  glass  screen.  The  seven  colours  of 
the  spectrum  coincide  in  the  focus,  and  there  is  formed  on  the 


mposition 


Light. 


355 


screen  a  perfectly  white  circular  image,  which  shows  that  the  union 

'its  of  the  spectrum  reproduces  Vhite  tight. 
II.  The  same  result  is  attained  by  replacing  the  double  convex 
lens  in  the  preceding  experiment  by  a  concave  mirror.     Th< 

.f  reflected  from  this  mirror,  there  is  formed 
in  the  i  ame  white  image  a 

>  of  Newton's  ci  lio\vn  that  th< 

of  the  spectrum  form  s  trdboard  * 


•ie  in  the  space  between  there  art- 

••urn.     Tli' 

e,  and  their  relative  dim; 
c  spectra  ( Jig,  285^).     V 
tated,  by  means  of  the  turniis^  table  r« 


356  On  Light.  [353 

it  appears  white,  or  at  all  events  of  a  greyish-white,  for  the  colours 
which  cover  it  cannot  be  arranged  exactly  in  the  same  dimensions 
or  of  the  same  tints  as  those  of  the  spectrum. 

To  explain  this  phenomenon  let  us  observe  that  the  impression 
produced  upon  the  eye  by  the  sight  of  a  luminous  body  lasts  a 
certain  time  after  the  cause  which  produced  it  has  ceased.  Thus, 
if  a  lighted  stick  be  rapidly  turned  round,  a  circle  of  light  is  pro- 
duced, which  shows  that  the  sensation  produced  upon  the  eye  lasts 
after  the  stick  has  passed  from  in  front  of  this  organ.  Thus,  too, 
in  the  above  experiment,  the  disc  is  turned  so  rapidly  that  the 
action  of  the  seven  colours  is  virtually  simultaneous,  and  the  eye  is 
affected  as  if  it  received  them  all  together,  and  the  disc  therefore 
appears  white. 

354.  Newton's  theory  of  the  composition  of  ligrht  and  the 
colour  of  bodies. — Newton  was  the  first  to  decompose  white  light 
by  the  prism,  and  to  recompose  it.  From  the  various  experiments 
which  we  have  described  he  concluded  that  white  light  was  not 
homogeneous,  but  formed  of  seven  lights  unequally  refrangible, 
which  he  called  simple  or  primitive  lights. 

He  was  further  led  to  the  conclusion,  that  bodies  are  not  of  them- 
selves coloured,  that  is  have  no  colour  of  their  own,  but  that  they 
have  the  property  of  decomposing  the  white  light,  which  illuminates 
them,  and  of  reflecting  unequally  the  various  kinds  of  light  of 
which  it  is  formed.  Thus,  vermilion  is  not  red  of  itself,  but  is 
endowed  with  the  property  of  reflecting  red  light  and  of  absorbing 
all  others,  or  at  any  rate  of  only  reflecting  them  in  far  less  propor- 
tion. In  like  manner  the  leaves  of  plants  are  not  truly  green  ;  they 
have  merely  a  greater  reflecting  power  for  green  than  for  any  other 
colour.  In  short,  bodies  are  only  coloured  by  the  light  they  re- 
flect. For,  let  these  same  green  leaves  be  placed  in  a  spectrum 
projected  in  a  dark  room,  if  they  are  in  the  green  band  they  will 
appear  of  a  dazzling  green,  far  brighter  than  their  natural  colour; 
but  if  they  are  placed  in  the  red  they  will  appear  red,  and  of  violet 
if  placed  in  violet.  A  similar  effect  is  produced  if  a  rose  be  suc- 
cessively placed  in  each  of  the  spectral  bands,  showing  that  the 
colour  of  a  body  is  not  peculiar  to  them,  but  depends  upon  the  kind 
of  light  which  their  molecular  constitution  gives  them  the  power  of 
reflecting.  In  speaking,  too,  of  the  green  or  the  red  pencil,  we  do 
not  mean  that  they  are  coloured  of  themselves,  but  merely  that  they 
have  the  power  of  producing  in  us  the  sensation  of  red,  or  of  yellow. 
The  eye  judges  colours  as  the  ear  judges  sounds  ;  both  the  colours 
and  the  sounds  depend  on  the  number  of  vibrations. 


-356]    Complementary  Colours.    Accidental  Images.     357 

Bodies  which  reflect  all  colours  in  the  spectrum  equally  well  arc 
white,  those  which  reflect  none  at  all  are  black  ;  so  that  black  is  not 
really  a  colour,  but  the  absence  of  colour. 

The  varied  shades  which  coloured  bodies  present  result  not 
merely  from  the  fact  that  they  simultaneously  reflect  various  kinds 
of  light,  but  reflect  them  to  different  extents.  Thus,  a  body  which 
reflects  yellow  and  blue  light  will  be  green,  but  a  green  the  shade 
of  which  varies  with  the  quantities  of  yellow  and  of  blue  light  which 
the  body  reflects.  If  by  means  of  an  opaque  screen,  part  or  all  of 
certain  colours  of  the  spectrum  be  intercepted,  and  the  others  be 
united  by  means  of  a  lens,  as  shown  in  fig.  284,  there  is  no  shade 
in  nature  which  cannot  be  reproduced,  but  with  a  lustre  and 
richness  of  colour  which  artifical  pigments  can  never  attain. 

355.  Colours  of  transparent  bodies. — We  have  seen  above  that 
opaque  bodies  owe  their  colour  to  the  power  of  decomposing  light 
by  reflection,  that  is,  of  reflecting  certain  colours  more  abundantly 
than  others.     It  is  owing  to  the  decomposition  of  light  that  trans- 
parent bodies  seem  to  be  coloured  :  though  here  the  decomposition 
is  effected  by  transmission  and  not  by  reflection.     If  all  rays  of  the 
spectrum  were  equally   transmissible  by   transparent  media  they 
would  necessarily  be  colourless  ;  that,  however,  is  never  quite  the 
case,  at  all  events  when  the  media  have  a  certain  thickness  ;  for  then 
they  absorb  certain  colours  of  the  spectrum  more  than  others,  and 
have  the  tint  of  the  more  transmissible  colour.     Water,  for  instance, 
seen  by  transmission  through  a  great  thickness,  has  a  greenish  tint, 
which  shows  that  of  all  colours  contained  in  white  light,  it  allows 
green  to  pass  most  easily. 

Air,  in  great  thickness,  gives  a  bluish  tint  to  distant  objects, 
which  would  rather  tend  to  prove  that  air  is  more  transparent  for 
blue  than  for  any  other  spectrum  colour.  It  is  more  probable  that 
this  effect  is  due  to  the  presence  of  the  aqueous  vapour  in  the  air. 

356.  Complementary    colours.     Accidental    images. — If   in 
white  light  any  colour  be  suppressed,  a  mixture  of  the  remainder 
is  called  the  complementary  colour,  for  it  is  the  colour  needed  to 
complete   the   sensation   of  white   light.     A  mixture  of  blue  and 
yellow  produces  a  green,  and,  accordingly,  green  is  the  comple- 
mentary  colour  to   red.     In   like  manner,  a  mixture  of  red  and 
yellow  produces  orange,  which  is  complementary  to  blue.     Simi- 
larly indigo  is  the  complementary  colour  to  yellow,  and  yellowish 
green  to  violet. 

Effects  of  complementary  colours  are  met  with  in  many  curious 


358  On  Light.  [356- 

experiments.  Thus,  let  any  coloured  object,  a  wafer  for  instance, 
be  placed  on  a  black  ground,  and  let  it  be  viewed  for  some  minutes 
until  the  sight  is  fatigued  ;  if  then  the  eyes  be  turned  to  a  sheet  of 
white  paper,  an  image  will  be  seen  of  the  same  form  as  the  object, 
but  of  the  complementary  colour ;  that  is,  that  if  the  wafer  is  red 
its  image  will  be  green,  if  it  is  orange  the  image  will  be  blue,  and 
so  forth.  In  like  manner,  if,  after  looking  for  some  time  at  the 
setting  sun,  the  eye  be  turned  to  a  white  wall,  an  intense  green 
disc  will  be  seen,  which  lasts  for  some.minutes,  after  which  the  red 
image  reappears  ;  a  second  green  image  succeeds  to  it,  and  so  on 
for  a  great  number  of  times,  until  the  appearance  fades  away. 

These  images  which  thus  persist  sometimes  after  an  object  has 
been  looked  at,  and  which  have  the  complementary  colours  of  those 
of  the  object,  were  first  observed  by  Buffon,  who  gave  them  the 
name  of  accidental  images. 

There  is  another  kind  of  accidental  colour  also  noticed  by  Buffon  ; 
when  a  coloured  object  placed  on  a  white  ground  is  attentively 
looked  at  for  some  time  the  object  is  seen  to  be  surrounded  by  a 
colour  which  is  complementary  to  that  of  the  object.  This  pheno- 
menon, which  is  known  as  the  accidental  halo,  is  easily  verified  by 
means  of  a  coloured  wafer  placed  on  a  sheet  of  white  paper. 

When  several  pieces  of  cloth  of  the  same  colour  are  successively 
looked  at  it  will  be  seen  that  the  latter  ones  appear  of  a  bad  shade. 
This  arises  from  the  fact,  that  the  accidental  colour  of  the  cloth 
begins  to  form,  and  its  own  tint  loses  its  brightness.  So  too  when 
designs  are  printed,  or  cloth  embroidered  on  a  coloured  ground, 
effects  may  be  obtained  quite  different  from  those  which  were  de- 
sired. Generally  if  two  adjacent  colours  are  complementary,  each 
will  acquire  a  greater  lustre  ;  but  if  they  are  of  the  same  tint,  they 
will  mutually  enfeeble  each  other.  It  will  thus  be  seen  how 
numerous  are  the  applications  which  the  phenomenon  of  accidental 
images  presents  in  combining  colours  in  pictures,  wall  papers, 
tapestry,  furniture,  even  the  toilet,  although  good  taste  has  long 
been  in  advance  of  the  data  of  science. 

357.  Irradiation. — This  is  a  phenomenon  in  virtue  of  which 
white  objects,  or  those  of  a  very  bright  colour,  when  seen  on  a  dark 
ground  appear  larger  than  they  really  are.  Thus  a  white  square 
upon  a  black  ground  seems  larger  than  an  exactly  equal  black  square 
upon  a  white  ground  fig.  286.  With  a  black  body  on  a  bright 
ground,  the  converse  is  the  case.  Again,  a  platinum  wire  made 
red-hot  by  the  passage  of  an  electrical  current  seems  far  thicker 


-  358]  Rainbow.  359 

than  it  is  in  reality.  Irradiation  arises  from  the  fact,  that  the 
impression  produced  on  the  retina  extends  beyond  the  outline  of 
the  image.  It  bears  the  same  relation  to  the  space 
occupied  by  the  image  that  the  duration  of  the 
impression  does  to  the  time  during  which  the 
image  is  seen. 

The  effect  of  irradiation  is  very  perceptible 
in  the  apparent  magnitude  of  stars,  which  may 
thus  appear  much  larger  than  they  really  are  ; 
also  in  the  appearance  of  the  moon  when  two  or 
three  days  old,  the  brightly  illuminated  crescent 
seeming  to  extend  beyond  the  darker  portion  of 


O 


the  disc,  and  hold  it  in  its  grasp.  Fig  286 

Plateau,  who  has  investigated  this  subject? 
finds  that  irradiation  differs  very  much  in  different  people,  and  even 
in  the  same  person  it  differs  on  different  days.  He  has  also  found 
that  irradiation  increases  with  the  lustre  of  the  object,  and  the  length 
of  time  during  which  it  is  viewed.  It  manifests  itself  at  all  dis- 
tances, diverging  lenses  increase  it,  condensing  lenses  diminish  it. 

358.  Rainbow. — The  rainbow  is  a  luminous  meteor  which  ap- 
pears in  the  clouds  opposite  the  sun  when  they  are  resolved  into 
rain.  It  consists  of  seven  concentric  arcs,  presenting  successively 
the  colours  of  the  solar  spectrum.  Sometimes  only  a  single  bow 
is  perceived,  but  there  are  usually  two  ;  a  lower  one,  the  colours 
of  which  are  very  bright,  and  an  external  or  secondary  one,  which 
is  paler,  and  in  which  the  order  of  the  colours  is  reversed.  In 
the  interior  rainbow  the  red  is  the  highest  colour  ;  in  the  other 
rainbow  the  violet  is.  It  is  seldom  that  three  bows  are  seen  ;  theo- 
retically a  greater  number  may  exist,  but  their  colours  are  so  feeble 
that  they  are  not  perceptible. 

The  phenomenon  of  the  rainbow  is  produced  by  the  decompo- 
sition of  the  white  light  of  the  sun  when  it  passes  into  the  drops, 
and  by  its  reflection  from  their  inside  face.  In  fact,  the  same  pheno- 
menon is  witnessed  in  dewdrops  and  in  jets  of  water  ;  in  short, 
wherever  solar  light  passes  into  drops  of  water  under  a  certain  angle. 

The  appearance  and  the  extent  of  the  rainbow  depend  on  the 
position  of  the  observer,  and  on  the  height  of  the  sun  above  the 
horizon  ;  hence  only  some  of  the  rays  refracted  by  the  rain-drops, 
and  reflected  in  their  concavity  to  the  eye  of  the  spectator,  are  ad- 
apted to  produce  the  phenomenon.  Those  which  do  so  are  called 
effective  rays. 


360 


On  Light. 


[358 


To  get  a  general  idea  of  this  let  us  refer  to  fig.  287,  in  which  two 
rain-drops,  a  and  c,  are  represented  extremely  magnified  as  com- 
pared with  the  arc  of  which  they  form  part.  The  pencil  of  white 
light  which  fall  upon  a,  is  refracted  on  entrance  into  the  droplet 
and  decomposed,  giving  rise  to  seven  rays,  red,  orange,  yellow, 
green,  blue,  indigo,  and  violet  (346).  At  the  point  <?,  on  the  posterior 
face  of  this  droplet,  a  portion  of  the  refracted  light  escapes,  and 
is  dispersed  in  the  atmosphere  without  giving  rise  to  any  particular 
phenomenon ;  the  light  which  has  not  emerged  from  the  droplet 
is  collected  at  a,  returns  and  emerges  in  being  a  second  time  re- 
fracted, and  reaches  the  observer's  eye  as  represented  in  the  figure. 

A  second  droplet,  c,  placed  below  the  preceding  one,  produces 
just  the  same  effect ;  yet  it  does  not  send  the  same  colour  to  the 
spectator.  For,  as  the  different  colours  are  unequally  refrangible,  the 
coloured  rays  which  emerge  from  the  same  rain-drop  diverge,  and, 
therefore,  are  not  propagated  together  ;  whence  it  follows  that  each 
drop  only  sends  one  kind  of  colour  tcwarc's  the  observer.  Put  from 
the  different  degree  of  refrangibility  of  each  ray,  the  droplets  on  the 


Fig.  287. 

outside  of  the  arc  send  only  red  rays  towards  the  eye,  and  those  on 
the  inside  violet  rays.  The  other  colours  arise  from  intermediate 
droplets. 

In  short,  the  rainbow  is  the  circumference  of  the  base  of  a  con.c 


-359]  Aberration  of  Refrangibility.  361 

the  apex  of  which  is  the  observer's  eye,  and  the  surface  of  this  cone 
is  formed  from  the  outside  to  the  inside  of  seven  successive  enve- 
lopes, red,  orange,  yellow,  etc.,  corresponding  to  each  of  the  bands 
of  the  spectrum.  The  nearer  the  sun  is  to  the  horizon  the  larger  is 
the  visible  part  of  the  rainbow  ;  but,  as  the  sun  rises,  the  arc  dimi- 
nishes, and  entirely  disappears  when  the  sun  is  42  degrees  above 
the  horizon.  Hence  the  rainbow  is  never  seen  except  in  the  morn- 
ing and  evening. 


CHAPTER   VI. 

INJURIOUS    EFFECTS   OF   COLOUR   IN    LENSES.      ACHROMATISM. 

359.  Aberration  of  refrangibility. — In  speaking  of  lenses  we 
have  been  silent  about  a  serious  defect  to  which  they  are  liable, 
which  is,  that  objects  seen  through  these  lenses  at  a  certain  distance 
seem  surrounded  by  an  iridescent  fringe,  which  fatigues  the  sight 
and  greatly  injures  the  precision  of  the  images. 

For,  as  lenses  may  be  compared  to  a  series  of  prisms  with  infi- 
nitely small  faces,  and  united  at  their  bases,  they  not  only  refract 
light,  but  also  decompose  it  like  a  prism.  On  account  of  this  dis- 
persion, therefore,  lenses  have  really  a  distinct  focus  for  each 
colour.  In  condensing  lenses,  for  example,  the  red  rays,  which  are 
the  least  refrangible,  form  their  focus  at  a  point  r  on  the  axis  of  the 
lens  (fig.  288),  while 
the  violet  rays,  which 
are  most  refrangible, 
coincide  in  the  nearer 
point,  v.  The  foci  of 
the  orange,  yellow, 
green,  blue,  and  indigo 

are      between       these  Fis-  288- 

points.  Hence  a  double  convex  lens  tends  to  give  seven  images, 
differently  cc  loured,  of  objects  seen  through  them.  These  images 
being  partly  superposed,  the  seven  colours  combine  in  the  centre 
to  form  white  light,  but,  on  the  contours,  the  extreme  colours  of 
the  spectrum  are  visible,  that  is,  more  especially  red  and  blue. 

This  injurious  colouration  of  the  images  is  called  the  chromatic 
aberration. 


362  On  Light.  [360- 

360.  Achromatic  lenses. — By  observing  the  phenomenon  of  the 
dispersion  of  colours  in  prisms  of  water,  of  oil  of  turpentine,  and  of 
crown-glass,  Newton  was  .led  to  suppose  that  dispersion  was  pro- 
portional to  refraction.  He  concluded  that  there  could  be  no  re- 
^  fraction  without  dispersion,  and  there- 
fore, that  achromatism  was  impossible. 
Almost  half  a  century  elapsed  before  this 
was  lound  to  be  incorrect.  Hall,  an 
Fig.  289.  English  philosopher,  in  1733,  was  the  first 
to  construct  achromatic  lenses,  but  he  did  not  publish  his  discovery. 
It  is  to  Dollond,  an  optician  in  London,  that  we  owe  the  greatest 
improvement  which  has  been  made  in  optical  instruments.  In  1757 
he  combined  two  lenses,  one  a  double  convex  crown-glass  lens,  the 
other  a  double  concave  lens  of flint-glass  (fig.  289),  a  kind  of  glass 
which  contains  a  good  deal  of  lead,  and  which  has  greater  disper- 
sive power  than  flint-glass. 

By  suitably  choosing  the  curvatures  of  these  two  lenses,  they 
may  become  unequally  dispersive,  and  as  the  dispersion  is  in  opposite 
directions,  one  of  the  lenses  being  convergent  and  the  other  diver- 
gent, two  effects  are  produced,  which  compensate  each  other  as  re- 
gards colouration,  but  not  as  concerns  refraction  ;  that  is,  a  ray  of 
white  light  which  has  traversed  such  a  lens  emerges  colourless,  but 
converging,  and  forming  a  single  focus  on  the  axis. 

The  lenses  thus  formed  of  flint  and  crown-glass  give  images 
which  are  not  coloured  on  the  edges  ;  they  have  hence  been  called 
achromatic  lenses  ;  achromatism  being  the  term  applied  to  the  phe- 
nomenon of  the  refraction  of  light  without  decomposition. 

361.  Spherical  aberration. — Chromatic  aberration  is  not  the 
only  defect  which  lenses  present  ;  they  have  another,  which  is 
known  as  spherical  aberration,  and  which  arises  from  the  fact,  that, 
apart  from  dispersion,  the  rays  which  traverse  a  condensing  lens 
do  not  exactly  coincide  in  a  single  focus.  Those  which  traverse 
the  lens  near  the  edges  are  too  much  refracted  as  compared  with 
those  which  traverse  the  central  part  ;  hence  the  former  rays 
converge  nearer  to  the  lens  than  the  latter,  in  consequence  of 
which  the  images  are  distorted. 

This  inconvenience  is  obviated  in  optical  instruments  by  inter- 
cepting the  rays  which  traverse  the  lens  near  the  edge  by  dia- 
phragms or  stops,  which  are  opaque  screens  perforated  by  circular 
holes,  and  which  only  allow  the  central  rays  to  pass. 


-362]        Different  kinds  of  Optical  Instruments.  363 


CHAPTER  VII. 

OPTICAL   INSTRUMENTS. 

362.  The    different  kinds   of  optical   instruments. — By   the 

term  optical  instrument  is  meant  any  combination  of  lenses,  or  of 
lenses  and  mirrors.  By  their  means  the  limits  of  vision  have  been 
enormously  increased,  and  the  most  favourable  influence  has  been 
exerted  on  the  progress  of  science,  by  opening  out  new  worlds  to 
investigation  which  would  otherwise  have  remained  unknown.  Opti- 
cal instruments  may  be  divided  into  three  classes  according  to  the 
ends  they  are  intended  to  answer,  viz.  : — i.  Microscopes,  which  are 
designed  to  obtain  a  magnified  image  of  any  object  whose  real  di- 
mensions are  too  small  to  admit  of  its  being  seen  distinctly  by  the 
naked  eye.  ii.  Telescopes,  by  which  very  distant  objects,  whether 
celestial  or  terrestrial,  may  be  observed,  iii.  Instruments,  designed 
to  project  on  a  screen  a  magnified  or  diminished  image  of  any 
object  which  can  thereby  be  either  depicted,  or  be  rendered  visible 
to  a  crowd  of  spectators  ;  such  as  the  camera  lucida,  the  camera  ob- 
scura,  photographic  apparatus,  the  magic  lantern,  the  solar  micro- 
scope, the  photo-electric  microscope,  etc.  The  two  former  classes 
yield  virtual  images,  the  last,  with  the  exception  of  the  camera 
lucida,  yield  real  images. 

General  composition  of  optical  instruments.  Of  the  various  in- 
struments enumerated  above,  those  of  the  first  two  groups  consist 
essentially  of  two  lenses  :  one  called  the  object  glass,  or  objective, 
receives  the  light  from  the  objects,  and  concentrates  it  in  a  focus 
where  it  gives  a  small  image  ;  the  other,  called  the  eyepiece,  or 
ocular,  acts  as  a  magnifying  glass,  is  near  the  eye,  and  serves  to 
view  the  image  formed  by  the  object  glass.  In  what  are  called  re- 
flecting telescopes,  a  concave  mirror  is  used  instead  of  an  object 
glass.  Generally  speaking  the  object-glass  and  the  eyepiece  are  not 
formed  of  a  single  glass,  but  of  several,  in  order  to  obtain  a  greater 
magnifying  power,  and  to  correct  chromatic  and  spherical  aberration 
(361).  These  glasses  are,  moreover,  mounted  in  long  metal  tubes, 
blackened  on  the  inside  so  as  to  absorb  the  oblique  rays,  which 
would  otherwise  injure  the  sharpness  of  the  image  ;  these  tubes 
can  further  be  slid  in  or  out  so  that  the  glasses  may  be  brought  to 
the  proper  distance. 


364 


On  Light. 


[363- 


363.  Galileo's  telescope. — Like  many  great  discoveries,  that 
of  the  telescope  seems  due  to  chance.  For  it  is  stated  to  have  been 
made  accidentally  by  the  children  of  a  Dutch  spectacle  maker  at 
Middlebourg.  Looking  at  a  vane  on  the  top  of  a  church  spire 
through  a  convex  and  concave  glass,  the  latter  being  nearer  the 
eye,  they  were  surprised  to  see  the  object  magnified,  and  apparently 
almost  within  reach.  The  father  repeated  the  experiment  and  ar- 
ranged the  two  glasses  in  tubes,  one  which  slided  in  the  other,  and 
thus  constructed  the  telescope. 

This  telescope  bears  Galileo's  name,  for  this  illustrious  astro- 
nomer was  the  first  to  direct  it  towards  the  heavens,  and  to  make 
astronomical  observations.  It  is  stated  that  he  was  at  Venice  when 
he  learnt  that  Zacharia  Jans  had  offered  to  Prince  Maurice  of 
Nassau  an  instrument  which  brought  objects  nearer  :  he  quickly 


Fig.  290. 

started  for  Padua,  where,  after  meditating  on  the  matter,  he  made 
some  experiments,  and  in  twenty-four  hours  rediscovered  the  tele- 
scope. 

The  telescopes  constructed  by  Galileo  were  gradually  improved 
from  a  magnifying  power  of  four  up  to  one  of  thirty  times.  By  its 
means  Galileo  discovered  the  mountains  of  the  moon,  Jupitor's 
satellites,  and  the  spots  on  the  sun.  From  these  numerous  dis- 
coveries he  acquired  the  name  Lyncens,  from  one  of  the  Argonauts, 
whose  sight  is  said  to  have  been  so  penetrating  that  he  could  see  to 
the  bottom  of  the  sea. 

Fig.  290  represents  the  arrangement  of  the  lenses  and  the  path 
of  the  rays  in  a  Galileo's  telescope.  The  object  glass,  O,  is  a  double 
convex,  while  the  eyepiece,  0,  is  a  double  concave  lens.  If  AB  is 
the  object  observed,  the  rays,  from  any  one  of  its  points,  A,  for  in- 
stance, tend  to  form  an  image  of  this  point  beyond  the  object-glass  ; 
but  meeting  the  double  concave  lens,  o,  these  rays  appear  divergent, 
and  seem  to  the  eye  which  receives  them  as  if  they  proceeded  from 


-363] 


Galileo's  Telescope. 


365 


the  point  a  ;  and  it  is  there  the  image  of  A  appears.  In  like  manner 
the  image  of  B  is  formed  at  b,  so  that  a  virtual  image,  ab,  is  formed, 
which  is  erect,  and  very  near. 

Galileo's  telescope  is  very  short  and  portable.    It  has  the  advan- 
tage of  showing  objects  in  their  right  position,  and  further,  as  it 


Fig.  291. 

has  only  two  lenses,  it  absorbs  very  little  light ;  in  consequence, 
however,  of  the  divergence  of  the  emergent  rays  it  has  only  a  small 
field  of  view,  and  in  using  it  the  eye  must  be  placed  very  near 
the  eyepiece.  The  eyepiece  can  be  moved  to  or  from  the  object 


366  On  Light.  [363- 

glass,  so  that  the  image  is  always  formed  at  the  distance  of  distinct 
vision. 

Opera  glasses  are  constructed  on  this  principle.  They  are 
usually  double,  so  as  to  produce  an  image  in  each  eye,  by  which 
greater  brightness  is  attained. 

364.  Astronomical  telescopes. — In  observing  the  stars  a  tele- 
scope with  two  condensing  lenses  is  used,  in  order  to  obtain  a  greater 
field  of  view.  Its  invention  is  due  to  Kepler,  and  it  is  known  as 
the  astronomical  telescope.  It  gives  reversed  images  of  objects,  but 
this  is  not  prejudicial  in  observing  the  stars. 

Fig.  291  represents  an  astronomical  telescope  with  a  cast  iron 
support,  and  mounted  with  a  hinge  motion  on  a  column  of  the 
same  metal ;  so  that,  not  only  can  any  degree  of  inclination  be 
imparted  to  it,  but  it  can  be  directed  to  any  part  of  the  horizon. 
By  means  of  a  handle  and  two  toothed  wheels  the  telescope  can 
be  raised  or  lowered  at  pleasure.  On  the  side  of  the  telescope  is 


Fig.  292. 

a  smaller  one  called  the  finder ;  for  as  it  magnifies  less  than  the' 
large  one,  it  embraces  a  greater  extent  of  sky,  and  therefore  is  more 
suited  for  finding  any  given  star,  which  is  then  observed  more 
minutely  with  the  large  glass. 

Fig.  292  represents  the  arrangement  of  the  glasses,  and  the  path 
of  the  rays  in  an  astronomical  telescope.  It  consists  of  two  double 
convex  glasses  ;  the  object  glass,  which  is  of  large  diameter,  and  but 
slightly  convergent,  gives  at  ab  a  reversed  and  very  small  image  of 
the  star  towards  which  the  telescope  is  directed.  This  image  is 
looked  at  through  the  eyepiece,  O,  which  acts  here  as  a  magnifying 
glass,  and  which,  for  that  purpose,  is  placed  so  that  the  image,  ab, 
is  formed  between  this  glass  and  the  principal  focus,-  F.  Thus  the 
observer  sees  a  reversed  and  greatly  enlarged  image  of  the  star  at 
cd. 

As  in  all  telescopes,  the  eyetube,  that  is,  the  tube  in  which  is  the 
eyepiece,  slides  in  the  other,  so  that  it  can  be  brought  nearer  or 
further  from  the  image,  ah,  which  can  thus  be  seen  at  the  distance 


-366]  Reflecting  Telescopes.  367 

of  distinct  vision.  In  powerful  telescopes  the  eyepiece  is  not  simple, 
as  in  the  above  case,  but  consists  of  a  greater  number  of  glasses, 
the  object  of  which  is  not  only  to  increase  the  magnifying  power, 
but  also  to  correct  spherical  and  chromatic  aberration. 

The  magnifying  power  of  a  telescope  is  greater  the  greater  the 
diameter  of  the  object  glass,  and  the  less  its  convexity  ;  and  the 
more  convex,  on  the  contrary,  is  the  eyepiece.  The  greatest  ob- 
stacle met  with  in  the  construction  of  these  telescopes  is  the 
difficulty  of  manufacturing  large  object-glasses. 

When  the  telescope  is  used  to  make  an  accurate  observation  of 
the  stars,  for  example,  their  zenith  distance,  or  their  passage  over 
the  meridiari,  a  cross  wire  is  added.  This  consists  of  two  very  fine 
metallic  wires  or  spider  threads  stretched  across  a  circular  aper- 
ture in  a  small  metal  plate.  The  wires  ought  to  be  placed  in  the 
position  where  the  inverted  image  is  produced  by  the  object  glass, 
and  the  point  where  the  wires  cross  .ought  to  be  on  the  optical 
axis  of  the  telescope,  which  thus  becomes  the  line  of  sight,  or 
collimation. 

365.  Terrestrial  telescopes. — The  terrestrial  telescope  differs 
from  the  astronomical  telescope  in  producing  images  in  their  right 


Fig.  293. 

positions.  This  is  effected  by  means  of  two  condensing  lenses, 
which  are  interposed  between  the  object  glass  and  the  eyepiece,  as 
seen  in.  fig.  293.  The  object  glass  forming  then,  at  I,  a  reversed 
image  of  the  object,  AB,  the  two  glasses,  m  and  «.,  impart  such  a 
direction  to,  the  rays  traversing  them,  that,  after  having  crossed 
between  the  two, glasses,  the  rays  reproduce  an  erect  image  at  /. 
The  eyepiece  acts  then  just  as  in  the  astronomical  telescope,  giving 
a  very  near,  erect,  and  magnified  image,  ab. 

The  terrestrial  telescope  is  sometimes  mounted  on  a  stand,  and 
sometimes  held  in  the  haad  ;  its  uses  are  too  well  known  to  need 
any  description. 

366.  Reflecting:  telescopes. — The  telescopes  previously  de- 
scribed are  refracting  or  dioptric  telescopes.  It  is,  however,  only 
in  recent... times. that  it  has  beeo.  possible  to  construct  achromatic 


368  On  Light.  [366- 

lenses  of  large  size  ;  before  this,  a  concave  metallic  mirror  was 
used  instead  of  the  object  glass.  Telescopes  of  this  kind  are  called 
reflecting  or  catoptric  telescopes.  The  principal  forms  are  those 
devised  by  Gregory,  Newton,  Herschel,  and  Cassegrain. 

Of  these  we  shall  describe  the  Newtonian  telescope,  which,  after 
long  disuse,  has  been  restored  to  favour,  in  great  measure  owing  to 
the  improvements  made  in  the  construction  of  the  concave  mirror 
used  in  it. 

Fig.  294  represents  the  section  of  a  Newtonian  telescope  as  modi- 
fied by  M.  Foucault,  and  fig.  295  a  perspective  view.  The  prin- 
cipal piece  of  the  telescope  is  a  concave  mirror,  M,  placed  at  the 
end  of  a  long  wooden  tube.  These  mirrors  were  formerly  of  metal, 
and  the  difficulty  of  working  such  mirrors,  so  as  to  give  them  a 
perfect  curvature,  was  so  great,  that  the  use  of  reflecting  telescopes 
was  virtually  abandoned. 

Foucault  having  discovered  a*  method  of  silvering  glass  without 


Fig.  294. 

injuring  its  polish,  and  as  glass  is  more  easily  worked  than  metal 
mirrors,  reflectors  for  telescopes  are  now  made  of  polished  glass, 
silvered  on  the  concave  surface  itself,  the  rays  of  light  which  come 
from  the  star  observed  are  there  reflected,  and  tend  to  form  at  the 
other  end  of  the  tube  a  real  and  very  small  image  of  the  star ;  but 
these  rays  fall  upon  a  small  rectangular  prism,  ;;/;/,  into  which  they 
pass  without  being  refracted,  and  form  with  the -large  face,  mn, 
such  an  angle  of  incidence  that  they  are  reflected  out  instead  of 
being  refracted  (330).  The  image  is  then  formed  at  ab,  in  front 
of  a  horizontal  tube,  in  which  are  a  series  of  magnifying  glasses, 
which  act  as  ocular,  and  give  of  the  image,  ab,  a  very  amplified 
virtual  image,  AB. 

Fig.  295  shows  how  the  instrument  is  worked.  The  right  hand 
of  the  observer  holds  a  handle  which  transmits  the  motion  to  an 
endless  chain,  and  this  to  two  other  chains,  which  pass  round 


-366] 


Reflecting  Telescopes. 


369 


pulleys,  and  enable  the  tube  to  be  more  or  less  inclined  ;  with  the 
left  hand  the  same  observer  turns  a  small  wheel,  fixed  to  a  screw, 
which  enables  him  to  move  slowly  the  front  part  of  the  apparatus 
in  a  lateral  direction,  so  that  he  can  follow  the  star  in  its  motion. 


Fig.  295. 

A  little  lower  than  the  eyepiece  and  above  the  small  wheel  is  a  milled 
head,  which  works  a  small  rack  and  pinion  motion  :  this  is  fixed 
to  a  movable  piece,  which,  at  the  same  time,  supports  the  prism, 
mn,  and  the  eyepiece  (fig.  294).  By  turning  this  milled  head  in 

B  B 


370 


On  Light. 


[366- 


either  direction,  the  prism  and  the  eyepiece  may  be  adjusted  until 
the  image,  AB,  is  formed  at  the  distance  of  distinct  vision  of  the 
observer. 

On  the  side  of  the  tube  is  a  smaller  telescope,  quite  similar  to 
the  large  one,  but  of  far  less  magnifying  power.  This  is  the  finder. 
From  its  small  magnifying  power,  not  more  than  ten,  it  embraces  a 
far  greater  extent  of  the  sky,  and  is  therefore  more  favourable  for 
finding  the  desired  star. 

367.  Herschel's  telescope. — SirW.  Herschel's  telescope,  which, 
until  lately,  was  the  largest  instrument  of  modern  times,  was  con- 
structed on  a  method  differing  from  those  described.  The  mirror 
was  so  inclined  that  the  image  of  the  star  was  formed  on  the 
side  of  the  telescope  near  the  eyepiece  (fig.  296) ;  hence  it  is  termed 


Fig.  296. 

^cat  front  view  telescope.  As  the  rays  in  this  telescope  only  undergo 
a  single  reflection,  the  loss  of  light  ;s  less  than  in  either  of  the  pre- 
ceding cases,  and  the  image  is  therefore  brighter.  The  magnifying 
power  is  the  quotient  of  the  principal  focal  distance  of  the  mirror 
by  the  focal  distance  of  the  eyepiece. 

Herschel's  great  telescope  was  constructed  in  1789;  it  was  40 
feet  in  length,  the  great  mirror  was  50  inches  in  diameter.  The 
quantity  of  light  obtained  by  this  instrument  was  so  great  as  to 
enable  its  inventor  to  use  magnifying  powers  far  higher  than  any- 
thing which  had' hitherto  been  attempted. 

Herschel's  telescope  has  been  exceeded  by  one  constructed  by 
the  late  Earl  of  Rosse.  This  magnificent  instrument  has  a  focal 
length  of  53  feet ;  the  diameter  of  the  mirror  is  6  feet,  and  it  weighs 
8,400  pounds.  It  is  at  present  used  as  a  Newtonian  telescope,  but 
'it  can  also  be  arranged  as  a  front  view  telescope. 


-369] 


Compound  Microscope. 


371 


MAGNIFYING  INSTRUMENTS. 

368.  Simple  microscope. — Microscopes  are  instruments  which, 
giving  very  magnified  images,  enable  us  to  observe  objects  which 
are  too  small  to  be  seen  with  the  naked  eye.  Two  kinds  are  dis- 
tinguished, the  simple  and  the  compound  microscope. 

The  first  of  these  is  nothing  more  than  a  small  highly  convergent 
lens,  which  is  used  as  a  magnifying  glass  as  seen  in  fig.  297.  The 
object  observed  is  then  placed  between  the  lens  and  its  principal 
focus,  and  the  magnifying  power  is  greater  the  more  condensing  is 
the  lens.  When  it  is  rather  large  it  is  mounted  in  horn  or  in  ivory, 
and  is  then  known  as  a  lens.  It  is  frequently  used  to  assist  the 


Fig.  297. 

sight  of  the  aged,  or  to  facilitate  certain  kinds  of  work,  which,  as  in 
watchmaking  and  engraving,  require  great  accuracy.  But  no  great 
magnification  is  thus  attainable,  and  in  order  to  observe  very  small 
objects,  the  compound  microscope  is  used,  which  is  so  called,  for  it 
is  made  up  of  several  lenses. 

369.  Compound  microscope. — Fig.  298  represents  the  mode  of 
using  a  compound  microscope,  and  fig.  299  the  path  of  the  luminous 
rays  in  the  interior  of  the  apparatus.  The  object  observed,  which 
is  always  very  small,  is  placed  at  a,  between  two  glass  plates  on  a 
support  called  the  stage.  OA0  is  a  brass  tube  in  which  are  two 
condensing  lenses,  the  object-glass,  o,  at  the  bottom,  and  the  eyepiece, 
O,  at  the  top.  The  object,  a,  being  placed  very  little  beyond  the 


372 


On  Light. 


[369- 


principal  focus  of  the  eyepiece,  we  know  that  a  real,  erect,  and 
greatly  magnified  image  will  be  formed  at  be  (343).  But  as  the 
eyepiece,  O,  is  at  such  a  distance  that  the  image,  be,  is  between  this 
glass  and  its  principal  focus,  F,  it  follows  that  for  an  eye  looking 


Fig.  298. 

through  it  the  eyepiece  acts  as  a  lens  (344),  and  gives  at  BC  a  vir- 
tual and  amplified  image  of  the  first  image,  be.  It  may  thus  be  said, 
that  the  compound  microscope  is  nothing  more  than  a  simple  mi- 


-  370]  Origin  and  Use  of  the  Microscope.  373 

eroscope  applied  not  to  the  object,  but  to  its  image  already  magni- 
fied by  the  first  lens. 

The  magnification  depends  more  especially  on  the  object-glass. 
In  order  to  increase  its  power  it  consists  of  two  or  three  small 
lenses,  superposed,  as  seen  in  H,  on  the  right  of  the  drawing  (fig.  299). 
To  the  eyepiece  a  second  glass  is  used,  the  object  of  which  is  less 
to  obtain  increased  magnification  than 
to  render  the  images  more  defined  by 
diminishing,  as  in  telescopes,  chro- 
matic and  spherical  aberration.  All 
the  glasses  are,  moreover,  achromatic. 
The  magnifying  power  in  compound 
microscopes  has  been  carried  to  1,800 
times,  and  even  more,  but  then  what 
is  gained  in  magnification  is  lost  in 
definiteness.  A  good  magnification 
does  not  exceed  600  in  length  and 
breadth,  which  amounts  to  a  super- 
ficial enlargement  of  360,000  times. 

From  the  great  magnification  of 
the  image  the  object  must  be  power- 
fully illuminated.  For  this  purpose, 
when  it  is  sufficiently  transparent,  it 
is  illuminated  from  below  by  a  con- 
cave mirror,  M,  which  concentrates 
upon  it  a  large  quantity  of  light,  as 
shown  in  fig.  299.  If  the  object  is 
opaque  it  is  illuminated  above  by  a  con- 
densing lens,  L  (fig.  298),  the  focus4  of 
which  is  formed  upon  the  object  itself. 

370.  Origin  and  use  of  the  mi- 
croscope. —  The  invention  of  the 
microscope  does  not  extend  further 
back  than  the  commencement  of  the 

seventeenth  century,  which  is  surprising,  for  it  had  long  been 
known  that  a  drop  of  water  placed  in  a  small  hole  in  a  thin  opaque 
plate  magnified  objects  seen  through  it.  From  the  commencement 
of  the  first  century,  A.D.,  the  philosopher  Seneca  announced  that 
writing  appeared  larger  under  a  glass  globe  containing  water. 
Finally,  in  the  thirteenth  century,  spectacles  were  used,  that  is, 
magnifying  glasses,  to  assist  the.  sight  of  the  aged.  The  inventor 


374  On  Light.  [370- 

of  the  microscope  is  not  known  ;  it  has,  probably,  only  acquired  its 
present  form  after  numerous  successive  improvements. 

The  microscope  has  been  the  origin  of  discoveries  in  the  vege- 
table and  animal  kingdom,  as  curious  as  they  are  varied.  Botanists 
owe  to  it  their  most  beautiful  discoveries  on  the  structure  of  the 
cellular  tissue  in  plants,  the  circulation  of  the  sap,  the  function  of 
leaves  in  the  respiration  of  vegetables.  In  entomology  it  has 
enabled  us  to  discover  a  crowd  of  small  animals  which  would  other- 
wise have  remained  unknown  from  their  extreme  minuteness.  Thus 
there  have  been  observed,  in  vinegar  and  in  sour  paste,  thousands 
of  small  organisms  called  -vibrions  ;  in  stagnant  waters  myriads  of 
animalcules,  as  remarkable  for  their  fantastical  forms  as  for  their 
beautiful  colours,  their  instincts,  their  warlike  or  sociable  manners. 
Mould  presents  the  appearance  of  small  mushrooms  with  the  most 
brilliant  colours.  In  short,  any  object  seen  through  the  microscope 
becomes  an  object  of  astonishment  and  admiration :  thus,  for  in- 
stance, a  hair,  a  piece  of  silk  thread,  the  eye  or  wing  of  a  fly,  a 
bee's  sting,  a  spider's  claw,  a  cat's  or  mouse's  hair,  the  down  of 
fruit,  the  scales  of  a  butterfly's  wing  or  offish,  starch  grains,  spider- 
web,  etc.,  etc.,  everywhere  we  recognise  the  infinite  perfection  of 
nature's  works. 

The  microscope  may  also  be  advantageously  used  to  recognise 
fraudulent  mixtures  in  cloths  of  various  kinds,  by  giving  a  means 
of  ascertaining  whether  they  contain  wool  or  silk,  linen  or  cotton. 


CHAPTER  VIII. 

OPTICAL  RECREATIONS. 

371.  Magic  lantern. — In  the  instruments  that  still  remain  to  be 
described,  the  object  is  to  project  on  a  screen  reduced  or  enlarged 
images  of  an  object,  so  as  to  exhibit  them  to  a  number  of  specta- 
tors, or  to  utilise  them  for  drawing. 

The  oldest  and  most  simple  of  these  apparatus  is  the  Magic 
lantern,  which,  as  everyone  knows,  is  one  of  the  first  physical  in- 
struments placed  in  children's  hands.  It  was  invented  by  Father 
Kircher,  a  German  Jesuit,  about  200  years  ago,  and  is  used  to  pro- 
ject a  magnified  image  of  small  objects  painted  on  glass  on  a  white 
screen  in  a  dark  room  (fig.  300).  It  consists  of  a  tin  plate  box 


-372] 


Phantasmagoria. 


375 


in  which  there  is  a  lamp  placed  in  the  focus  of  a  concave  mirror,  M 
(fig.  301).  The  reflected  rays  fall  upon  a  condensing  lens,  L,  which 
concentrates  them  on  the  figure  painted  on  a  glass  plate,  ad.  There 
is  a  system  of  two  lenses,  ;;/,  acting  as  a  single  one  of  great  magni- 
fying power,  at  a  distance  from  ab  of  rather  more  than  its  focal 
distance.  At  this  distance  the  system  of  two  lenses  acts  as  in  the 
experiment  (fig.  300)  ;  that  is,  a  real  and  very  much  magnified 
image  of  the  figure  on  the  glass  is  produced  on  the  screen.  The 


Fig.  300. 

image  is  made  erect  by  placing  in  the  lantern  the  glass  painted 
in  such  a  manner  that  the  design  is  reversed.  The  image,  AB,  is 
formed  at  so  much  the  greater  distance,  and  is  so  much  the  more 
amplified,  the  nearer  the  glass,  ab,  is  to  the  principal  focus  of  the 
system  of  lenses,  ;;/,  and  the  -greater  the  magnification  of  this 
system. 

372.  Phantasmagoria. — This  is  only  a  modification  of  the 
magic  lantern,  and  dates  from  the  end  of  the  eighteemh  century  :  its 
name  is  derived  from  two  Greek  words,  which  signify  assemblage  of 


376 


On  Light. 


[372- 


phantoms,  for  it  was  originally  used  tD  produce  fright,  by  making 
spectres  appear  in  darkness. 

The  internal  arrangement  of  the  phantasmagoria  is  just  the  same 
as  in  the  magic  lantern,  the  only  difference  being,  that  in  the  magic 
lantern  the  image  projected  on  the  screen  is  always  of  the  same 
size,  while,  in  the  case  of  the  phantasmagoria,  the  size  may  be  varied 
at  pleasure.  To  understand  how  this  is  effected,  let  us  refer  to  fig. 
301,  which  represents  the  arrangement  of -glasses  in  the  magic  lan- 
tern. The  lenses,  ;;/,  which  are  used  to  project  the  images  on  the 
screen,  being  always  at  the  same  distance  from  the  painted  glass, 
ab,  the  image,  AB,  is  always  at  the  same  distance,  and  is  always 
therefore  of  the  same  size.  Now  if  one  of  the  lenses,  ///,  be  brought 


Fig.  301. 


nearer  the  glass,  ab,  it  follows  from  the  properties  of  lenses  (343) 
that  the  image  will  be  formed  at  a  greater  distance,  and  will  be 
larger.  Hence  the  effect  sought  requires  two  movements  ;  one 
which  brings  the  system  of  lenses,  ;;/,  nearer  the  painted  glass,  to 
amplify  the  image  ;  the  other,  which  makes  the  whole  apparatus  re- 
cede, so  that  the  image,  while  being  moved  away,  is  always  formed 
upon  the  same  screen  as  at  first. 

To  obtain  this  double  effect  the  whole  apparatus  is  mounted  upon 
•four  small  wooden  wheels  covered  with  cloth,  so  that  they  roll 
noiselessly  on  the  floor.  Figure  302  represents  a  phantasmagoria 
thus  arranged,  with  the  difference  that  in  the  figure  it  is  double,  that 
is,  consists  of  two  apparatus  united.  We  shall  presently  (373)  see 
the  reason  for  this  double  use,  and  for  the  moment  we  shall  only 
consider  one  of  the  parts.  The  front  of  the  box  is  provided  with  a 
conical  brass  tube  :  in  this  tube  is  the  lens  of  projection,  which  is 


-372] 


Phantasmagoria. 


377 


not  fixed,  but  may  be  advanced  or  receded  by  means  of  a  milled 
head  and  screw,  which  the  experimenter  turns  with  the  hand. 


378  On  Light.  [372- 

A  large  white  sheet  is  stretched  in  front  of  the  apparatus,  and 
the  spectators  are  on  the  other  side  of  the  sheet.  The  whole  being 
in  profound  darkness,  the  experimenter  is  careful  first  of  all  to  keep 
the  projecting  lens  away  from  the  glass,  on  which  are  painted  the 
objects  he  desires  to  show.  Thus  there  is  at  first  formed  on  the 
sheet  a  very  small  image  of  the  object.  Then,  with  one  hand,  the 
experimenter  brings  the  lens  near  the  painted  glass,  while  with  the 
other  he  draws  towards  himself  the  apparatus,  and  away  from  the 
cloth  ;  the  image  projected  on  the  latter  gradually  increases,  and 
ultimately  becomes  very  large.  But  the  spectators,  who  are  pre- 
vented from  seeing  whether  the  position  of  the  image  changes  or 
not,  and  who  see  the  image  very  distinctly  through  the  cloth,  fall 
into  the  illusion  that  its  increase  in  size  is  due  to  its  coming  nearer 
them.  Some  authors  have  supposed  that  use  was  made  of  the 
phantasmagoria  in  remote  antiquity,  and,  by  means  of  apparatus  of 
this  kind,  those  initiated  into  the  mysteries  of  I  sis  and  Ceres  were 
terrified,  and  the  infernal  deities  evoked  were  made  to  appear.  Yet 
nothing  indicates  that  lenses  were  then  known  ;  concave  mirrors, 
however,  would  be  sufficient  for  producing  effects  analogous  to  those 
of  the  phantasmagoria. 

373.  Polyorama,  or  dissolving:  views. — 1\\z  polyorama,  is  an 
application  of  the  phantasmagoria.  It  is  double,  as  represented 
in  fig.  302,  and  the  two  systems  of  lenses  converge  towards  the 
same  point  of  the  cloth  which  receives  the  image.  Two  pictures 
on  glass  are  used  representing  the  same  view  under  different 
conditions  ;  for  example,  Mount  Vesuvius  seen  at  daytime,  calm, 
and  with  a  slight  cloud  of  smoke  rising  from  it  ;  the  other  when 
seen  at  night  vomiting  forth  flames  and  torrents  of  fiery  lava. 
Having  arranged  these  glasses,  each  in  one  of  the  phantasmagoria, 
and  the  lenses  being  so  arranged  as  to  project  the  magnified  images 
on  exactly  the  same  part  of  the  cloth,  the  diaphragm  of  the  one  con- 
taining the  picture  representing  the  effect  of  day  is  opened  ;  the 
other  remaining  closed.  Then  when  the  image  has  for  some  time 
been  exposed  to  the  view  of  the  spectators,  a  mechanism,  <7,is  worked, 
which  gently  closes  the  one  which  has  been  exposed,  and  opens 
the  other.  It  follows,  that  in  gradually  passing  through  all  the 
shades  of  light,  the  image  which  produces  the  effect  of  day  disap- 
pears, while  it  is  gradually  succeeded  by  the  effect  of  night  repre- 
sented on  the  other.  In  like  manner,  too,  the  effect  of  the  moon 
rising  may  be  made  to  succeed  to  sunset ;  to  a  calm  and  transpa- 
rent sea,  a  tempest ;  to  a  smiling  landscape,  a  snow  effect,  and  so 


-374]  Photo-Electric  Microscope.  379 

forth.     Hence  the  name  polyorama,  from  two  Greek  words,  which 
signify  several  views. 

374.  Photo-electrical  microscope. — This  apparatus  is  based  on 
the  same  principles  as  the  magic  lantern  and  the  phantasmagoria. 
But,  as  in  these  apparatus,  the  subjects  painted  on  glass  are  of 
some  size,  no  great  enlargement  is  required,  and  therefore  the  illumi- 


Fig.  303. 

nation  need  not  be  very  intense.  Whereas  objects,  the  image  ot 
which  is  reproduced  by  the  photo-electrical  microscope,  being  very 
small,  should  be  considerably  magnified,  and  the  light  must  there- 
fore be  very  powerful,  or  else  the  image  will  be  confused  and  indis- 
tinct. Hence  the  apparatus  is  illuminated  by  the  powerful  light 
which  the  electric  battery  yields. 


380  On  Light.  [374- 

Figure  303  represents  the  use  of  the  photo-electric"  microscope. 
At  the  base  of  the  vessel  is  a  series  of  vessels  which  serve  for  the 
disengagement  of  electricity,  and  which  we  shall  afterwards  describe 
as  the  electric  battery .  From  these  vessels  the  electricity  passes  by 
two  stout  copper  wives  to  two  rods  of  charcoal,  contained  in  the  box 
B.  Thus  charged  with  electricity,  these  carbons  become  heated  to 
incandescence,  and  emit  such  a  bright  light  that  the  eye  cannot 
support  it.  A  reflector,  I,  sends  the  luminous  rays  in  the  direction 
of  the  tube,  C,  where  they  meet  two  condensing  lenses,  which  concen- 
trate them  on  the  very  small  object  which  is  to  be  magnified,  and 
which  is  arranged  between  two  glass  plates,  X.  The  rays  pass  from 
thence  into  a  tube  D,  where  there  is  a  system  of  condensing  lenses 
intended  to  produce  the  same  effect  of  projection  as  the  lenses*,  »z,  in 
the  magic  lantern  (371)  ;  that  is,  it  is  a  system  of  lenses  which  pro- 
duces on  a  white  screen  at  a  distance  an  extremely  magnified  image 
of  the  small  object  placed  between  the  glass  plates.  The  tube,  D,  is 
movable,  and  may  be  approached  to  or  receded  from  the  object, 
so  as  to  vary  the  magnification. 

In  the  adjacent  figure,  the  image  projected  on  the  screen  is  that 
of  the  infusoria  which  are  found  in  paste  when  it  has  fermented.  A 
small  quantity  is  mixed  with  water,  and  a  few  drops  put  in  a  small 
glass  box  with  parallel  faces,  which  is  placed  at  X.  A  multitude  of 
these  animalcules  are  seen  on  the  screen,  ten  or  twelve  inches  in 
length,  which  move  about  in  a  confused  mass,  and  soon  die  in  con- 
sequence of  the  heat  which  is  concentrated  along  with  the  light  in 
the  focus  of  the  lenses. 

375.  Experiments  with  tlie  photo-electric  microscope  are 
among  the  most  interesting  in  the  whole  range  of  physics.  By  its 
means,  objects  of  extreme  minuteness  may  be  exhibited,  greatly 
magnified,  to  a  large  number  of  spectators.  A  hair,  for  example, 
looks  like  a  broomstick  ;  a  flea  like  a  sheep  ;  the  itch-tick,  an  ani- 
malcule found  in  itch  pustules,  and  by  which  this  disease  is  propa- 
gated, appears  like  a  man's  head  ;  the  same  is  the  case  with  the 
animalcules  found  in  decayed -cheese,  although  these  cannot  be  seen 
by  the  naked  eye.  One  of  the  most  remarkable  experiments  is  that 
showing  the  circulation  of  the  blood.  This  is  made  by  placing  be- 
tween two  glass  plates  the  tail  of  a  living  tadpole,  that  is  to  say,  the 
young  of  a  frog,  before  its  upper  and  lower  limbs  are  developed. 
There  is  then  observed  on  the  screen  a  kind  of  illuminated  map,  all 
the  rivers  in  which  appear  to  flow  very  rapidly  :  this'  is  the  blood 
which  circulates  in  the  veins.  A  very  beautiful  experiment  is  the 


-376] 


Camera  Obscura. 


381 


crystallisation  of  salts,  and  especially  of  sal  ammoniac.  This  salt  is 
dissolved  in  water,  and  a  drop  of  the  solution  is  spread  on  a  glass 
plate,  which  is  placed  in  the  apparatus.  As  the  heat  makes  the 
water  evaporate,  a  vegetation  quickly  forms,  which  is  surprising 
from  the  promptitude  with  which  the  crystalline  molecules  group 
themselves  together  to  produce  magnificent  ramifications  like  fern 
leaves . 

The  apparatus  wre  have  described  is  sometimes  modified,  so  as  to 
be  illuminated  by  sunlight,  and  is  then  called  the  solar  microscope. 
it  is  also  illuminated  by  the  intense  light  produced  by  allowing  the 
oxy hydrogen  flame  to  impinge  upon  a  piece  of  lime.  It  is  then 
called  the  oxyliydrogen  microscope, 

376.  Camera  obscura. — A  Neapolitan  physician,  Jean  Baptiste 


Fig.  304. 

Porta,  first  observed  in  1680,  that  if  a  very  small  hole  be  perforated 
in  the  shutter  of  a  dark  room,  one  that  is  quite  deprived  of  light,  all 
objects  which  can  reach  the  hole  depict  themselves  on  the  opposite 
screen,  and  of  so  much  the  smaller  dimension  the  nearer  this  screen 
is  to  the  aperture. 

Porta  also  found,  that  by  fixing  a  double  convex  lens  in  the  aper- 
ture, and  placing  a  white  screen  in  the  focus,  the  image  was  much 

* 


382 


On  Light. 


[376- 


brighter,  and  more  definite.  In  both  cases  the  images  are  inverted. 
Fig.  304  shows  how  images  formed  in  the  camera  obscura  are  re- 
versed upon  the  screen.  It  is  due  to  the  rays  crossing  on  entering 
the  aperture.  It  follows  in  fact,  that  rays  from  the  higher  parts  ot 
the  object  proceeding  in  a  straight  line  meet  the  lower  part  of  the 
screen,  while  the  reverse  is  the  case  with  rays  from  the  lower  part. 
The  colouration  of  the  image  is  jeadily  understood  by  observing, 
that  the  reflected  rays  are  of  the  same  colour  as  the  reflecting  body ; 


Fig.  305. 


that  is,  that  a  red  body  reflects-  red  rays,  a  yellow  body  yellow  rays, 
and  so  on  ;  each  portion  of  the  image  is  formed  by  the  coincidence 
of  rays  of  the  same  colour  as  the  corresponding  part  of  the  object 
it  represents. 

The  images  formed  in  the  camera  obscura  have  the  peculiarity 
of  being  independent  of  the  shape  of  the  aperture  through  which 
the  rays  enter  provided  this  is  very  small,  that  is,  that  whether  this 
aperture  is  round,  square,  or  triangular,  the  image  formed  on  the 
screen  is  always  a  faithful  reproduction  of  external  objects,  and  not 


-377] 


Images  of  the  Camera  Obscura. 


383 


of  the  hole  made  in  the  shutter.  To  account  for  this  phenomenon 
let  us  consider  the  case  of  a  pencil  of  solar  light  passing  into  a  dark 
room  of  any  shape  whatever  (fig.  305).  From  the  magnitude  of  the 
sun  this  hole  is  really  nothing  more  than  a  point  ;  whence  it  follows, 
that  the  whole  of  the  rays  which  traverse  it  represents  an  immense 
luminous  cone,  of  which  the  hole  is  the  summit  and  the  sun  the 
base.  By  their  being  prolonged  into  the  chamber  these  rays  give 
rise  to  a  second  cone  resembling  the  first,  but  far  smaller  ;  and  if 
this  second  cone  falls  upon  a  screen  which  is  perpendicular  to  the 
straight  line  joining  its  summit  to  the  centre  of  the  sun,  it  produces 


Fig.  306. 

on  this  screen  a  circular  image  like  the  sun.  If  the  screen  is 
obliquely  inclined  towards  this  line,  as  represented  in  fig.  305,  the 
image  is  elongated,  but  it  never  has  the  shape  of  the  aperture  unless 
the  screen  is  very  close. 

In  the  same  manner  we  must  explain  the  luminous  circles  former 
on  the  ground  under  an  avenue  of  trees  illuminated  by  the  sun, 
whatever  be  the  shape  of  the  spaces  on  the  foliage  through  which 
the  light  passes,  a  circular  image  of  the  sun  is  projected  upon  the 
ground  (fig.  306.) 

377.  Rectification  of  images  of  the  camera  obscura. — When 


On  Light.  [377- 

in  a  camera  obscura  a  monument  or  a  landscape  is  to  be  reproduced, 
the  image  must  be  rectified.  For  this  purpose  the  apparatus  is  ar- 
ranged as  in  fig.  307.  A  little  above  the  hole  through  which  the 
light  enters  a  plane  mirror  is  placed,  inclined  so  as  to  send  the  rays 
towards  a  condensing  lens  fixed  at  the  end  of  a  tube.  Below  this 
lens,  and  at  its  focus,  is  placed  a  white  screen,  on  which  external 


Ffg.  307. 

objects  depict  themselves.  The  images  thus  obtained,  rectified  by 
the  reflection  of  the  rays  from  the  plane  mirror  and  their  passage 
through  the  lens,  are  not  merely  admirable  from  their  fidelity  and 
colour,  but  they  do  what  no  other  kind  of  reproduction  can  do,  they 
reproduce  motion.  If  the  camera  obscura  is  set  up  in  front  of  a 
promenade,  or  a  public  place,  the  images  of  the  passers-by  are  seen 


-378] 


Portable  Camera  Obscura. 


385 


to  move  across  the  screen  with  such  fidelity  that  they  can  be  re- 
cognised. 

The  camera  obscura  gives  in  this  manner  an  amusing  spectacle  ; 
it  may,  moreover,  be  used  in  drawing,  for  even  a  person  who  can- 
not draw  can  trace  with  a  pencil  the  outlines  of  the  image,  and 
trace  it  on  a  screen.  For  this  latter  purpose  the  following  arrange- 
ment is  usually  adopted. 

378.  Portable  camera  obscura. — To  make  views  by  means  of 
the  camera  obscura,  it  should  be  light  and  portable,  and  should  not 
occupy  too  large  a  space.  Fig.  308  represents  a  simple  and  con- 


Fig.  308. 

venient  form  of  the  apparatus.  It  consists  of  a  wooden  tripod, 
supporting  a  board  of  the  same  material,  and  surrounded  by  a  cur- 
tain which  forms  a  small  tent,  in  which  the  artist  places  himself. 
In  the  centre  of  the  tent  is  a  small  table  resting  on  a  tripod,  on 
which  is  produced  the  image.  At  the  top  of  the  apparatus,  in  a 
brass  tube  open  at  the  side,  is  a  glass  prism,  which  produces  the 
effect  both  of  the  inclined  mirror  and  ot  the  lens  in  the  camera 
obscura  described  above.  For  this  purpose  the  first  face  of  the 
prism  is  convex,  as  represented  in  fig.  309.  Hence  on  passing 

C  C 


386  On  Light.     ^^^  [378- 

into  this  prism  the  rays  from  a  distant  object  converge  ;  then  under- 
going a  total  reflection  on  the  side  m  (330),  they  are  sent  towards 
the  third  face,  which  is  concave,  whence  they  emerge  with  the 
same  degree  of  convergence  that  they  had  before  traversing  the 


Fig.  309. 

lens,  and  there  is  thus  reproduced  at  ab  on  the  table  P  the  image 
of  the  object  AB,  from  which  they  come.  The  designer  takes  then 
the  outlines  of  this  image  on  a  sheet  of  paper. 

PHOTOGRAPHY. 

379.  Photography. — Photography  is  the  art  of  fixing  the  images 
of  the  camera  obscura  on  substances  sensitive  to  light.  The  various 
photographic  processes  may  be  classed  under  three  heads  :  photo- 
graphy on  metal,  photography  on  paper,  and  photography  on  glass. 

Wedgwood  was  the  first  to  suggest  the  use  of  chloride  of  silver 
in  fixing  the  image  ;  and  Davy,  by  means  of  the  solar  microscope, 
obtained  images  of  small  objects  on  paper  impregnated  with  chloride 
of  silver ;  but  no  method  was  known  of  preserving  the  images  thus 
obtained,  by  preventing  the  further  action  of  light.  Niepce,  in 
1814,  obtained  permanent  images  of  the  camera  by  coating  glass 
plates  with  a  layer  of  a  varnish  composed  of  bitumen  dissolved  in 
oil  of  lavender.  This  process  was  tedious  and  inefficient,  and  it 
was  not  until  1839  that  the  problem  was  solved.  In  that  year, 
Daguerre  described  a  method  of  fixing  the  images  of  the  camera, 
which,  with  the  subsequent  improvements  of  Talbot  and  Archer, 
has  rendered  the  art  of  photography  one  of  the  most  marvellous 
discoveries  ever  made,  either  as  to  the  beauty  and  perfection  of 
the  results,  or  as  to  the  celerity  with  which  they  are  produced.  Fig. 
310  gives  a  vertical  section  of  the  kind  of  camera  obscura  used  by 


-379] 


Photography. 


387 


photographers.  It  consists  of  a  rectangular  wooden  box  in  two 
pieces,  one  of  which,  C,  is  fixed,  and  the  other,  Bj  can  be  drawn  in 
or  out  like  a  drawer.  In  the  front  of  the  box  is  a  brass  tube,  A,  in 
which  is  a  condensing  lens,  L,  which  is  fixed.  In  A  is  a  second 
tube  which  can  be  moved  backwards  or  forwards  by  .a  rack  and 
pinion  moved  by  the  milled  head,  D.  In  this  second  tube  is  a 
second  lens,  L',  which,  by  the  motion  of  the  tube,  is  brought  nearer 
or  further  from  the  lens,  L.  The  combination  of  the  two  lenses 
forms  what  is  called  an  object  glass  with  combined  lenses.  The 
advantage  of  this  arrangement  is  that  it  works  more  rapidly  than 
an  object  glass  with  a  single  lens,  has  a  shorter  focal  distance,  and 
can  be  more  readily  focussed. 

On  the  face  of  the  box  opposite  the  object  glass  is  a  screen  of 


i 

K 

m 

•. 

Fig.  310. 

ground  glass,  E,  which  can  be  removed  at  will,  and  on  which  a  re- 
versed image  of  the  object  is  formed.  Thus,  if  a  portrait  is  to  be 
taken  the  person  is  placed  at  a  distance  of  three  or  four  yards  from 
the  camera,  which  is  then  adjusted  until  the  image  is  formed  in  the 
proper  position  on  the  glass.  It  is  then  placed  in  exact  focus  by 
sjpwly  approaching  or  removing  the  lens,  L'.  The  glass  is  con- 
tained in  a  frame  which  can  be  easily  removed  and  replaced  by 
the  slide  containing  the  material  on  which  the  photograph  is  to  be 
taken. 

The  photographs  on  metal,  or  daguerreotype,  so  called  from  Dar 
guerre  the  inventor,  are  now  seldom  used.  The  photographic 
methods  in  glass  and  paper  are  infinitely  varied,  not  as  regards  the 
optical  part,  but  as  concerns  the  substances  employed,  and  there- 
fore as  regards  the  chemical  reactions  involved.  We  will  content 
ourselves  with  describing  the  ordinary  method  of  taking  a  portrait 
on  paper. 

c  c  2 


388  .  %  On  Light.  [379- 

For  this  purpose  what  is  called  a  negative  must  first  be  taken — 
an  inverse  image  of  the  object,  that  is  to  say,  in  which  the  light 
parts  are  dark,  and  vice  versa.  With  this  view  a  glass  plate  is 
coated  with  a  thin  layer  of  collodion  (gun  cotton  dissolved  in  ether), 
containing  a  certain  quantity  of  iodide  of  potassium.  This  plate 
thus  coated  is  then  placed  in  a  solution  of  nitrate  of  silver.  By 
the  chemical  reaction  between  the  iodide  of  potassium  and  the 
nitrate  of  silver  a  coating  of  iodide  of  silver  is  formed  on  the  plate, 
which  is  sensitive  to  light,  and  hence  the  operation  must  be  per- 
formed in  a  dark  room.  The  plate  is  then  placed  in  the  slide,  and 
inserted  in  the  camera  instead  of  the  focussing-glass.  The  slide  is 
so  constructed  that  the  plate  can  be  instantaneously  exposed  to  or 
cut  off  from  the  action  of  light.  After  exposure  for  a  suitable  time 
the  slide  is  removed  to  a  dark  room.  No  change  is  visible  in  the 
plate,  but  on  pouring  over  it  a  solution  called  the  developer,  an 
image  gradually  appears.  The  principal  substances  used  for  deve- 
loping are  protosulphate  of  iron  and  pyrogallic  acid.  The  action 
of  light  on  iodide  of  silver  produces  some  change,  in  virtue  of 
which  the  developers  have  the  property  of  reducing  to  the  metallic 
state,  those  parts  of  the  iodide  of  silver  which  have  been  most 
acted  upon  by  the  light.  When  the  picture  is  sufficiently  brought 
out,  water  is  poured  over  the  plate,  in  order  to  prevent  the  further 
action  of  the  developer.  The  parts  on  which  light  has  not  acted 
are  still  covered  with  iodide  of  silver,  which  would  also  be  affected 
if  the  plate  were  now  exposed  to  the  light.  It  is,  accordingly, 
washed  with  solution  of  hyposulphite  of  sodium,  which  dissolves 
the  iodide  of  silver  and  leaves  the  image  unaltered.  The  picture 
is  then  coated  with  a  thin  layer  of  spirit-varnish,  to  protect  it  from 
mechanical  injury. 

When  once  the  negative  is  obtained,  it  may  be  used  for  printing 
an  indefinite  number  of  positive  pictures.  For  this  purpose,  papfr 
is  impregnated  with  chloride  of  silver,  by  immersing  it  first  in 
a  solution  of  chloride  of  sodium  and  then  in  one  of  nitrate  of  silver  ; 
chloride  of  silver  is  thus  formed  on  the  paper  by  double  decompo- 
sition. The  negative  is  placed  on  a  sheet  of  this  paper  in  a  copying 
frame,  and  exposed  to  the  action  of  light  for  a  certain  time.  The 
chloride  of  silver  becomes  acted  upon — the  light  parts  of  the  nega- 
tive being  most  affected,  nnd  the  dark  parts  least  so.  A  copy  is 
thus  obtained,  on  which  the  lights  of  the  negative  are  replaced  by 
shades,  and  conversely.  In  order  to  fix  the  picture,  it  is  washed  in 
a  solution  of  hyposulphite  of  sodium,  which  dissolves  the  unaltered 


-380]  Positives  on  Glass.  389 

chloride  of  silver.     The  picture  is  afterwards  immersed  in  a  bath  of 
chloride  of  gold,  which  gives  it  tone. 

380.  Positives  on  glass. — Very  beautiful  positives  are  obtained 
by  preparing  the  plates  as  in  the  preceding  cases  ;  the  exposure  in 
the  camera,  however,  is  not  nearly  so  long  as  for  the  negaives. 


Fig.  311. 

The  picture  is  then  developed  by  pouring  over  it  a  solution  of 
protosulphate  of  iron,  which  produces  a  negative  image  ;   and  by 


390  @n  Light.  .  [380- 

afterwards  pouring  a  solution  of  cyanide  of 'potassium  over  the 
plate,  this  negative  is  rapidly  converted  into  a  positive.  It  is  then 
washed  and  dried,  and  a  coating  of  varnish  poured  over  the  picture. 

381.  Diorama. — The  name  diorama  is  derived  from  two  Greek 
words  which  signify  viewed  through,  and  is  applied  to  pictures 
painted  on  muslin  or  on  calico,  so  as  to  represent  two  opposite 
effects  like  the  polyorama,  according  as  the  pictures  are  seen  by  re- 
flection or  by  transmission. 

The  picture  is  arranged  vertically  in  a  dark  room  as  represented 
in  fig.  311.  The  first  effect,  that  painted  on  the  front  of  the  cloth, 
is  illuminated  by  reflection  :  the  second,  that  painted  behind,  is  illu- 
minated by  transmission.  With  this  view  light  enters  through  a 
window,  M,  in  an  upper  story,  and  is  sent  to  a  screen,  E,  which  re- 
flects it  towards  the  picture,  and  lights  it  from  the  front ;  behind  the 
picture  is  another  window,  N,  which,  when  open,  lights  it  behind. 
The  shutters.  NN,  being  closed,  the  spectators  first  see  the  subject 
on  the  front  of  the  cloth.  By  a  simple  arrangement,  a  shutter,  A, 
which  slides  without  noise  in  two  grooves,  is  made  to  advance,  and 
when  the  picture  is  scarcely  illuminated,  by  degrees  the  shutters, 
NN,  are  opened;  and  then  the  picture  painted  on  the  other 
side  of  the  cloth  appears  through  it,  and  is  substituted  for  the 
former  one. 

The  diorama  was  invented  by  Daguerre,  who  had  great  skill  in 
this  kind  of  painting.  The  above  figure  represents  -the  valley  of 
Goldau  before  the  terrible  landslip,  which  took  place  on  September 
2,  1806.  At  the  moment  light  was  intercepted  by  the  screen,  light- 
ning flashed,  thunders  roared,  and  there  were  all  the  effects  of  a 
violent  storm.  On  the  return  of  day,  the  rocks  had  given  way,  the 
lake  had  been  partly  filled  up,  and  the  chalet  destroyed  ;  in  short, 
the  image  of  ruin  and  desolation  was  reproduced  with  astounding 
fidelity. 

382.  Ghost  scenes. — We  will  give  here  a  description  of  a  curious 
optical  effect,  which  was  first  introduced  some  years  ago  in  the 
theatres,  under  the  name  of  ghost  scenes. 

In  order  the  more  readily  to  understand  the  appearance  of  these 
spectres,  let  us  recal  an  effect  which  everyone  has  observed.  When 
towards  evening  on  a  railway  we  look  at  the  windows  of  carriage 
doors,  we  see  a  pale  and  indistinct  image  of  the  travellers  inside. 
This  is  an  effect  of  reflection  from  the  panes,  which  reflect  the  light 
that  illuminates  persons  and  objects  placed  in  the  compartment  ; 
and  the  faint  light  of  the  images  arises  from  the  fact,  that  the  panes 


_382]  Ghost  Scenes.  39* 

allowing  great  part  of  the  light  to  be  transmitted,  send  very  little 
towards  the  observer.  A  similar  effect  is  produced  when  in  the 
evening,  in  a  well-lighted  street,  a  window  front,  which  is  little  or 


392  On  Light.  [382- 

not  at  all  lighted,  is  looked  at.  The  observer  sees  his  own  image 
and  that  of  the  passengers  on  the  other  side  of  the  panes.  These 
effects  are  not  seen  in  full  daylight,  for  the  images  which  tend  to  be 
reproduced  are  effaced  by  the  brightness  of  the  light. 

These  effects  have  been  utilised  in  public  to  simulate  the  ap- 
pearance of  ghosts.  Fig.  312  represents  the  arrangement  of  the 
apparatus  intended  for  this  use.  On  the  floor  of  the  stage,  and  not 
visible  by  the  spectators,  is  an  actor  covered  by  a  sheet,  and  in- 
tended to  represent  the  ghost.  Between  the  actor  and  the  public  is 
a  dark  lantern,  in  which  is  the  lime  light,  which  gives  an  extremely 
bright  light.  An  assistant  directs  the  light  upon  the  actor,  and  the 
white  cloth,  thus  powerfully  illuminated,  sends  its  rays  towards  an 
inclined  sheet  of  glass,  placed  near  the  assistant.  This  glass,  which 
is  silvered,  sends  almost  all  the  reflected  light  towards  a  second 
sheet,  which  is  not  silvered,  on  the  stage  itself.  This  latter  plate 
acts  like  those  in  carriages  and  in  shopwindows,  which  we  have 
mentioned  above,  and  being  traversed  by  the  greater  part  of  the 
incident  rays,  sends  but  little  light  towards  the  spectator.  Yet,  as 
during  this  time,  care  is  taken  that  the  illumination  in  the  room  is 
very  faint,  the  light  is  sufficient  to  give  a  cloudy  image  of  the  actor 
placed  under  the  stage. 

If  another  actor  enters  the  scene  the  public  see  very  distinctly 
through  the  glass,  which  is  carefully  concealed  by  hangings  and 
decorations  ;  and  if  this  actor  is  behind  the  plate  at  the  same  dis- 
tance as  the  image,  he  can  join  his  action  with  that  of  the  ghost, 
and  produce  a  complete  illusion. 

The  same  effects  are  produced  with  a  single  plate,  but  as  its 
obliquity  tends  to  give  inclined  images,  to  rectify  them,  the  actor 
under  the  theatre  must  hold  himself  so  much  inclined  as  to  render 
his  play  very  difficult.  With  the  two  sheets  represented  in  the 
above  figure,  the  actor  retains  his  natural  position. 

VISION   AND   STEREOSCOPE. 

383.  Structure  of  the  eye  and  mechanism  of  vision. — Al- 
though the  description  of  the  eye  belongs  to  physiology  rather  than 
to  physics,  we  may  give  an  account  of  this  organ,  which  is  not 
merely  a  true  optical  instrument,  but  one  of  great  perfection  ;  it 
has,  for  instance,  the  remarkable  property  of  adapting  itself  at  once 
to  see  distinctly  at  all  distances,  which  the  best  optical  instruments 
could  not  do. 


-383]  The  Eye  and  Mechanism  of  Vision.  393 

The  eye  is  almost  spherical  in  shape,  and  is  surrounded  by 
several  membranes,  which  fig.  313  represents  open  from  back  to 
front.  The  front  part  of  the  eye  is  a  perfectly  transparent  membrane, 
c,  called  the  transparent  cornea,  and  which  is  commonly  called  the 
'white  of  the  eye.  At  a  small  distance  behind  the  cornea,  is  a  mem- 
branous diaphragm,  hi,  called  the  iris  ;  it  constitutes  the  variously 
coloured  disc  which  appears  in  the  middle  of  the  white  of  the  eye, 
and  to  which  is  due  the  colour.  In  the  centre  of  the  iris  is  an 
aperture  called  the  pupil ;  in  man  this  is  circular,  and  in  the  cat 
narrow  and  elongated,  and  through  it  rays  pass  into  the  eye.  Be- 
hind the  iris,  but  very  near  it,  is  the  crystalline,  o,  which  is  a  trans- 
parent mass,  having  the  shape  and  fulfilling  the  functions  of  a 
double  convex  lens.  The  whole  of  the  back  part,  from  the  crystal- 
line to  the  back  of  the  eye,  is  filled  with  a  gelatinous  transparent 


Fig.  313- 

mass,  like  white  of  egg,  which  is  called  the  "vitreous  humour.  In 
front  of  the  eye,  between  the  crystalline  and  the  cornea,  is  a  per- 
fectly transparent  liquid  called  the  aqueous  humour.  The  whole 
of  the  back  inside  part  of  the  eye  is  lined  with  a  soft,  whitish,  trans- 
parent membrane,  R,  called  the  retina  ;  it  is  nothing  more  than  the 
extension  of  a  nerve,  N,  which  proceeds  to  the  brain,  and  transmits 
the  sensation  of  vision,  whence  it  receives  the  name  optic  mrve. 
Behind  the  retina  is  a  second  membrane,  C,  called  the  choroid, 
•which  is  impregnated  by  a  black  matter,  that  absorbs  all  rays  which 
should  not  cooperate  in  producing  vision.  Lastly,  a  membrane,  S, 
the  sclerotica,  surrounds  the  whole  eyeball  behind,  and  joins  the 
transparent  cornea  in  front. 

These  details  being  known,  we  may  easily  account  for  the  me- 
chanism of  vision  ;  for  the  eye  is  nothing  more  than  a  small  camera 
obscura  (376),  of  which  the  pupil  is  the  aperture,  the  crystalline  is 
the  condensing  lens,  and  the  retina  is  the  screen  on  which  the 


394  On  Light.  [383- 

image  is  formed.  Thence  the  optic  nerve,  carrying  to  the  brain 
the  impression  produced  by  the  vibrations  of  the  ether  on  the  ner- 
vous system  of  the  retina,  enables  us  to  perceive  external  objects. 
In  accordance  with  this  explanation,  we  should  see  objects  reversed, 
and  not  in  their  natural  position.  The  inversion  of  images  in  the 
eye  has  greatly  occupied  both  physicists  and  physiologists,  and 
many  theories  have  been  proposed  to  explain  how  it  is  that  we  do 
not  see  inverted  images  of  objects.  Some  have  supposed  that  it  is 
by  custom,  and  by  a  regular  education  of  the  eye,  that  we  see  objects 
in  their  true  position,  that  is  to  say,  in  their  position  relative  to  us. 
The  visual  impression  becomes  corrected  by  the  impression  of 
other  senses,  such  as  that  of  touch.  Miiller,  Volkmann,  and  others 
contended  that,  as  we  see  everything  inverted,  and  not  simply  one 
object  among  others,  nothing  can  appear  inverted,  because  terms  of 
comparison  are  wanting.  It  must,  however,  be  admitted  that  none 
of  these  theories  is  quite  satisfactory. 

384.  Distance  of  distinct  vision.  Snort  and  long:  sight. — 
We  know  that  in  double  convex  lenses  the  distance  of  images  from 
the  lens  increases  or  diminishes  as  the  object  is  approached  or  re- 
ceded (338).  Hence/according  to  the  distance  of  the  objects 
looked  at,  it  would  seem  that  the  image  formed  by  the  crystalline 
should  be  sometimes  formed  exactly  on  the  retina,  and  sometimes  a 
little  in  front  of  or  behind  this  membrane.  Only  objects  placed  at 
a  certain  distance  should  then  be  seen  distinctly  ;  all  those  nearer 
or  further  should  appear  confused.  This  does  not  occur  with  a  well- 
shaped  eye,  for  it  sees  objects  distinctly  at  very  different  distances  ; 
whence  it  is  concluded,  that  the  eye  has  the  power  of  equally  accom- 
modating itself,  so  that  the  image  is  always  formed  exactly  upon 
the  retina. 

Yet,  though  the  eye  can  well  distinguish  objects  at  very  different 
distances,  there  is  in  the  case  of  each  person  a  distance  at  which 
objects  are  more  distinctly  seen  than  at  any  other.  This  distance 
is  called  the  distance  of  distinct  vision ;  it  varies  with  different  per- 
sons, and  often  in  different  eyes  in  the  same  individual ;  for  small 
objects  like  print  it  is  usually  about  ten  or  twelve  inches. 

Those  who  can  only  see  well  at  shorter  distances  have  a  defect 
in  the  shape  of  the  eye  ;  they  are  said  to  be  myoptic  or  short-sighted, 
from  two  Greek  words  which  signify  close  the  eyes  ;  for  myoptic 
persons,  in  order  to  see  more  distinctly,  do  in  fact  involuntarily  halt 
close  the  eyes.  If  the  distance  of  distinct  vision  is  greater  than  ten 
or  twelve  inches,  that  is  also  due  to  a  malformation  of  the  eye,  and 


-384]  Short  and  Long  Sight.  395 

those  affected  by  it  are  called  long-sighted  or  presbyoptic,  from  a 
Greek  word  which  signifies  aged,  for  this  defect  is  usually  met  with 
in  aged  persons. 

Myopy,  or  short-sight,  results  from  too  great  a  convexity  of  the 
cornea,  or  of  the  crystalline.  The  eye  being  too  convergent,  the  rays 
of  light  are  refracted  in  such  a  manner,  that  instead  of  forming  their 
focus  exactly  on  the  retina,  they  form  it  a  little  in  front,  and  there- 
fore the  image  which  this  membrane  perceives  is  confused.  But 
if  objects  are  approached  to  the  eye,  the  image  recedes,  and  is 
formed  exactly  on  the  retina,  when  the  objects  are  sufficiently  near, 
which  explains  why  short-sighted  persons  only  see  objects  when 
they  are  very  close.  They  can  also  see  more  distinctly  by  con- 
tracting the  pupil,  or  by  looking  through  a  small  hole  perforated 
in  a  card  ;  for  then,  as  the  diameter  of  the  luminous  pencil  which 
penetrates  into  the  eye  is  less,  the  rays  mainly  penetrate  the  crys- 
talline at  the  centre,  and  being  therefore  less  affected  by  its  excess 
of  convexity,  they  form  the  focus  at  a  greater  distance.  Myopy  is 
mainly  met  with  in  young  people  ;  as  they  grow  older,  the  convexity 
of  the  eye  diminishes,  so  that  their  sight  generally  becomes  better, 
when  that  of  other  people  becomes  worse. 

Presbytism,  or  long-sight,  is  due  to  the  flattening  of  the  crystalline, 
as  the  eye  is  then  no  longer  sufficiently  convergent,  the  rays,  instead 
of  forming  their  focus  on  the  retina,  tend  to  form  beyond  it,  whence 
it  arises  that  the  eye  only  observes  a  confused  object.  But  as  the 
objects  recede,  the  image  comes  nearer  the  crystalline,  and  is  ulti- 
mately formed  exactly  on  the  crystalline,  when  objects  are  suffi- 
ciently distant  ;  which  explains  why  long-sighted  persons  only  see 
objects  when  they  are  distant. 

Short-sight  is  remedied  by  the  use  of  concave  or  diverging  lenses 
before  the  eyes  ;  as  the  pencil  is  spread  out  before  entering  the 
eye,  by  the  action  of  these  glasses,  the  focus  of  the  crystalline  is 
receded  as  far  as  the  retina,  provided  the  degree  of  divergence  of 
the  glasses  is  suitably  adapted  to  the  excess  of  convexity  of  the  crys- 
talline. For  far-sight,  on  the  contrary,  condensing  or  convex 
lenses  should  be  used,  so  as  to  correct  the  want  of  convexity  of  the 
crystalline.  As  the  rays  then  become  more  convergent  before 
entering  the  eye,  the  image,  which  would  otherwise  be  formed 
beyond  the  retina,  approaches  it,  and  is  ultimately  formed  exactly 
upon  it. 

For  a  long  time  double  concave  lenses  were  exclusively  used  for 
short-sight,  and  double  convex  for  far-sight.  We  may  mention, 


396 


On  Light. 


[384- 


however,  the  concavo-convex  lenses  represented  in  O,  in  fig.  262 
for  long  sight,  and  those  in  R  (fig.  263)  for  short  sight.  These 
are  called  periscopic glasses,  from  two  words  meaning  to  see  round  \ 
for,  as  their  shape  better  enables  them  to  embrace  the  eyeball,  they 
facilitate  vision  in  all  directions  ;  and,  as  they  do  not  deform  ob- 
jects, they  do  not  fatigue  the  eye  like  other  glasses. 

385.  Binocular  vision. — Although  we  have  two  eyes,  and  when 
we  fix  them  on  the  same  object,  each  forms  its  own  image  upon  the 
retina,  yet  we  only  see  one  object,  just  as  with  two  ears  we  only  hear 
one  sound.     Various  hypotheses  have  been  made  to  account  for 
single  vision  with  two  eyes.     Some  have  considered  it  as  an  effect 
of  habit ;  others  assigning  to  it  a  physiological  cause,  have  assumed 
that  two  points  similarly  placed  on  the -two  retinas  correspond  to 
the  same  nervous  filament  which,  coming  from  the  brain,  bifurcates 
towards   each   eye.     Hence   the    two   impressions   simultaneously 
produced  on  the  two  retinas  result  in  a  single  sensation. 

Not  only  does  simultaneous  vision  with  two  eyes  enable  us  to  see 
bodies  with  greater  lustre,  but  it  gives  us  the  impression  of  relief,  as 
the  stereoscope  well  shows. 

386.  Stereoscope. — The  stereoscope,  so  called  from  two  Greek 


Fig- 


Fig.  3I5- 


words  which  IOVX&  perception  of  solidity,  is  an  ingenious  instrument, 
which  was  invented  by  Sir  C.  Wheatstone,  and  modified  to  its 
present  form  by  Sir  D.  Brewster. 

To  understand  the  effect  of  the  stereoscope,  let  us  observe  that 
when  we  look  at  an  object  with  two  eyes,  each  eye  does  not  see  it 
under  exactly  the  same  aspect,  but  under  a  slightly  different  per- 
spective. Thus  let  a  small  object,  such  as  a  dice,  be  successively 
viewed  with  one  eye,  at  a  slight  distance,  without  moving  the  head. 
If  the  cube  be  just  in  front  of  the  observer,  looking  at  it  with  his 
left  eye,  he  will  see  this  face,  and  a  small  portion  of  the  left  side, 
the  other  being  concealed  (fig.  314) ;  if,  on  the  contrary,  he  views  it 
with  his  right  eye,  he  sees  the  front  and  a  portion  of  the  right  side, 


-386] 


Stereoscope. 


397 


the  other  being  hidden  (fig.  315).  Thus  the  two  images  formed  on 
the  retina  are  not  quite  identical,  for  each  corresponds  to  a  different 
point  of  view.  It  is  this  difference  in 
the  images  which  gives  us  the  sensa- 
tion of  relief  in  bodies,  and  enables 
us  to  appreciate  their  shape  and  their 
distance. 

This  may  be  confirmed  by  making 
two  drawings  of  the  same  object, 
taken  respectively  with  the  perspective 
belonging  to  the  right  and  to  the  left 
eye  ;  then,  as  each  eye  looks  separately 
at  the  drawing  through  prisms  or 
lenses,  which  makes  the  two  drawings 
coincide,  by  giving  the  rays  of  light 
the  same  direction  as  if  they  con- 
verged from  a  single  object,  the  im- 
pressions produced  upon  each  retina 
will  be  the  same  as  if  the  object  itself 
were  viewed.  The  illusion  is  in  fact 
so  complete,  that,  however  prejudiced 
we  may  be,  it  is  impossible  not  to  be  deceived,  so  true  are  the 
effects  of  relief  and  perspective. 


mm, ' 


Fig.  317- 


398 


On  Light.  , 


[386- 


This  is  the  principle  of  the  stereoscope.  Fig.  316  shows  the  path 
of  the  rays  of  light  in  the  instrument.  At  A  is  the  drawing  of  the 
object  seen  with  the  left  eye ;  at  B  that  of  the  same  object  seen 
with  the  right.  The  rays  from  these  images  fall  on  two  lenses  m 
and  n,  and  take,  after  having  traversed  them,  the  same  direction  as 
if  they  came  from  the  point  C  ;  the  object  represented  at  A  and  B 
appears  in  relief  at  this  spot. 

The  lenses  m  and  n  must  impart  exactly  the  same  deviation  to  the 
rays,  and  they  should  therefore  be  exactly  identical.  Brewster 


Fig.  318. 

attained  this  result  by  cutting  in  two  halves  a  double  convex  lens, 
and  placing  the  right  half  in  front  of  the  left  eye,  and  the  left  half 
in  front  of  the  right  eye,  as  shown  in  fig.  316. 

To  produce  the  sensation  of  relief,  the  two  dissimilar  images  at  A 
and  B  should  give  from  two  different  points  of  view  so  faithful  a 
representation  of  the  same  object,  that  these  separate  views  cannot 
be  taken  by  the  hand.  And  it  is  only  practicable  by  means  of 
photography.  Fig.  317  represents  two  photographs  of  a  statuette 


-386]  ..  Stereoscope.  39^ 

of  Franklin,  taken  at  a  slightly  different  angle.  That  of  the  left 
represents  more  of  the  full  face,  and  must  be  looked  at  with  the 
left  eye  ;  the  other  one  is  more  in  profile,  and  must  be  viewed  with 
the  right  eye.  These  two  views  being  placed  in  the  stereoscope 
disappear  for  each  observer,  for  then  the  two  sensations  special  to 
each  eye  coalesce,  and  form  a  single  image  as  represented  in  fig. 
318,  and  the  original  appears  so  solid,  with  such  perfect  relief,  that 
the  imagination  can  with  difficulty  realise  the  fact  that  we  are  only 
concerned  with  a  drawing  on  a  plane  surface. 


40O  On  Magnetism.  [387- 


BOOK   VII, 

ON    MAGNETISM. 

CHAPTER   I. 
PROPERTIES  OF  MAGNETS. 

387.  Natural  and  artificial  magnets. — Natural  magnet,  or 
loadstone,  is  a  mineral  which  has  the  property  of  attracting  iron 
and  a  few  other  metals,  especially  nickel  and  cobalt.  This  mineral 
is  an  oxide  of  iron,  that  is,  a  compound  of  iron  and  oxygen  like  rust, 
from  which  it  only  differs  chemically  in  containing  rather  less  oxygen. 

Loadstone  has  another  property,  which  is  not  less  remarkable, 
namely,  that  when  it  is  balanced  on  a  pivot,  or  suspended  to  a 
thread,  it  points  towards  a  certain  direction  of  the  horizon  ;  and  by 
this  property  this  mysterious  stone,  which  is  of  a  dull  brown  colour, 
and  has  no  lustre,  deserves  a  place  above  the  most  valuable  pre- 
cious stones,  for,  like  a  new  Ariadne's  thread,  it  guides  mariners  in 
darkness,  and  enables  them  to  steer  with  the  same  certainty  on  sea 
as  on  a  travelled  road. 

This  loadstone,  or  magnetic  stone,  was  known  to  the  ancients, 
who  called  it  Lydian  stone,  or  stone  Magnesia  ;  for  it  was  first  found 
near  a  village  of  this  name  in  Lydia.  And  from  the  town  of  Mag- 
nesia the  Greeks  derived  the  name  magnes,  from  which  is  derived 
the  word  magnetism,  under  which  name  philosophers  understand 
the  whole  of  the  properties  which  magnets  possess.  Magnetic  iron 
ore  is  very  abundant  in  nature  ;  it  is  met  with  in  the  older  geo- 
logical formations,  especially  in  Sweden  and  Norway,  where  it  is 
•worked  as  an  iron  ore,  and  furnishes  the  best  quality  of  iron. 

Besides  natural  there  are  also  artificial  magnets,  so-called  from 
their  being  produced  by  art.  They  are  usually  made  of  steel. 
When  steel  is  tempered,  that  is,  when  it  is  raised  *o  a  high  tem- 
perature, and  suddenly  cooled  by.  being  immersed  in  cold  water, 


-387J 


Natural  and  A  rtificial  Magnets. 


401 


it  acquires  great  hardness  ;  and  it  is  in  virtue  of  this  property  that 
it  becomes  so  valuable  for  cutting  instruments.  Steel  has  not 
naturally  the  power  of  attracting  iron  ;  but  when  it  is  tempered 
and  made  hard  this  property  may  be  imparted  to  it  by  rubbing  it 
with  a  powerful  magnet  ;  and  it  then  becomes  itself  a  magnet. 

Artificial  magnets  have  just  the  same  properties  as  natural  ones  ; 
but  are  far  more  powerful  and  convenient  ;  they  are,  accordingly, 
generally  used  in  experiments.  They  are  sometimes  made  into 


Fig.  319- 

bars  a  foot  or  two  long,  like  that  represented  in  fig.  319  ;  some- 
times in  a  horseshoe  form  (fig.  329)  ;  or  lastly,  if  they  are  to  be 
movable,  they  are  cut  out  of  a  thin  sheet  in  the  shape  of  a  lozenge  as 
shown  in  fig.  321.  A  small  agate  cup  is  let  into  the  centre  in  such 
a  manner  that  the  needle  can  rest  on  a  vertical  pivot  and  oscillate 
freely  in  a  vertical  plane.  Thus  arranged,  the  artificial  magnet 
becomes  a  magnetic  needle. 

D  D 


402 


On  Magnetism. 


[388- 


388.  Distribution  of  magnetic  force  in  magnets. — The  force 
with  which  a  magnet  attracts  iron  is  not  everywhere  the  same. 
The  greatest  attraction  is  at  the  ends  ;  it  decreases  rapidly  from 
there  towards  the  middle,  where  there  is  no  attraction.  For,  if  a 
magnetised  bar  is  immersed  in  iron  filings,  when  it  is  withdrawn 
the  filings  are  seen  to  adhere  to  the  end  in  long  and  compact  fila- 
ments (fig.  319),  but  not  a  particle  adheres  to  the  middle. 

The  two  points  near  the  ends  where  the  attraction  is  most 
powerful  are  called  the  poles  of  the  magnet,  and  the  medial  part, 
where  there  is  no  attraction,  is  called  the  neutral  line.  All  magnets, 
natural  or  artificial,  have  each  two  poles  and  a  neutral  line.  Some- 


Fig.  320. 


times,  besides  the  two  principal  poles,  there  are  observed  inter- 
mediate poles,  which  are  called  consequent  poles.  This  arises 
from  some  inequality  in  the  temper  of  the  steel,  or  in  the  manner 
in  which  the  bar  has  been  magnetised.  We  shall  always  assume 
that  magnets  have  only  two  poles. 

The  action  of  magnets  upon  iron  is  exerted  through  all  bodies 
which  are  not  magnetic.  Thus,  a  magnet  being  placed  on  a  table 
and  a  piece  of  cardboard  rested  upon  it,  iron  filings  are  allowed  to 
fall  through  a  sieve  (fig.  320).  As  the  filings  fall,  acted  upon  by  the 
two  poles,  they  become  arranged  in  long  filaments,  which  group 
themselves  along  curved  lines  from  one  pole  to  the  other  ;  but  over 
the  middle  of  the  magnet  no  action  is  observed,  and  the  filings  are 
arranged  as  they  would  be  upon  any  other  substance. 


-389]   Laws  of  Magnetic  Attraction  and 'Repulsion.    403 

389.  laws  of  mag-netic  attraction  and  repulsion. — When 
the  two  poles  of  a  magnet  are  compared  as  to  the  action  they  exert 
upon  iron,  they  seem  to  be  completely  identical ;  this  identity  is, 
however,  only  apparent :  for,  if  to  the  same  pole  of  a  magnetic 
needle  (fig.  321)  the  two  poles  of  a  bar  magnet  held  in  the  hand  be 
successively  presented,  the  curious  phenomenon  is  observed  that, 
if  the  pole  a  of  the  needle  is  attracted  by  pole  B  of  the  bar,  it  is 
on  the  contrary,  repelled  by  the  other  pole  of  the  latter ;  which 


Fig.  321. 

shows  that  the  poles  of  the  bar  are  not  exactly  identical,  for  one 
repels  the  pole  a,  while  the  other  attracts  it.  The  same  difference 
may  be  ascertained  to  exist  between  the  two  poles  of  the  needle 
ab ;  for  if  the  same  pole,  B,  of  the  bar  be  successively  presented 
to  the  two  ends  of  the  movable  needle,  in  one  case  there  is  re- 
pulsion, in  the  other  attraction. 

We  shall  presently  see  that  a  freely  suspended  magnet  always 
sets  with  one  pole  pointing  to  the  north,  and  the  other  towards  the 
south.  The  end  pointing  towards  the  north  is  called  in  this  country 
the  north  fiole,  and  the  other  end  is  the  south  pole.  The  end  of 
the  magnetic  needle  pointing  to  the  north  is  sometimes  called  the 
marked  end  of  the  needle. 

Hence,  in  reference  to  magnetic  attractions  and  repulsions  the 
following  law  may  be  enunciated  : 

D  D  2 


404  On  Magnetism.  [389- 

Poles  of  the  same  name  repel,  and  poles  of  contrary  name  attract 
one  another. 

The  opposite  actions  of  the  north  and  south  poles  maybe  shown 
by  the  following  experiment  : — A  piece  of  iron,  a  key  for  example, 
is  supported  by  a  magnetised  bar  (fig.  322).  A  second  magnetised 
bar  of  the  same  dimensions  is  then  moved  along  the  first,  so  that 
their  poles  are  contrary.  The  key  remains  suspended  so  long  as 
the  two  poles  are  at  some  distance,  but  when  they  are  sufficiently 
near,  the  key  drops  just  as  if  the  bar  which  supported  it  had  lost  its 
magnetism.  This,  however,  is  not  the  case,  for  the  key  would  be 
again  supported  if  the  first  magnet  were  presented  to  it  after  the 
removal  of  the  second  bar. 


Fig.  322. 

The  attraction  which  a  magnet  exerts  upon  iron  is  reciprocal, 
as  is  easily  verified  by  presenting  a  mass  of  iron  to  a  movable 
magnet,  when  the  latter  is  attracted. 

390.  Hypothesis  of  two  magnetic  fluids. — In  order  to  explain 
the  phenomena  of  magnetism,  the  existence  of  two  hypothetical 
magnetic  fluids  has  been  assumed,  each  of  which  acts  repulsively 
on  itself,  but  attracts  the  other  fluid.  The  fluid  whose  resultant 
effects  predominate  at  the  north  pole  of  the  magnet  is  called  the 
north  or  boreal  fluid,  and  that  at  the  south  pole,  the  south  or  austral 
fluid.  Sometimes  the  terms  positive  and  negative  are  employed, 
corresponding  to  the  north  and  south  fluids. 

It  is  assumed  that,  before  magnetisation,  these  fluids  are  com- 
bined round  each  molecule,  and  mutually  neutralise  each  other ; 
they  can  be  separated  by  the  influence  of  a  force  greater  than  that  of 
their  mutual  attraction,  and  can  arrange  themselves  in  a  certain 
definite  position  about  the  molecules  to  which  they  are  attached, 
but  cannot  be  removed  from  them. 

The  hypothesis  of  the  two  fluids  is  convenient  in  explaining 
magnetic  phenomena,  and  will  be  adhered  to  in  what  follows.  But  it 
must  not  be  regarded  as  anything  more  than  an  hypothesis,  and  it 


-391]  Hypothesis  of  Magnetic  Fluids.  405 

will  afterwards  be  shown  that  magnetic  phenomena  can  also  be  ex- 
plained by  assuming  that  they  result  from  electrical  currents, 
circulating  in  magnetic  bodies. 

39 1 .  Influence  of  magnets  upon  magnetic  substances. — Mag- 
netic substances  are  substances  containing  the  two  magnetic  fluids, 
but  in  the  neutral  state,  that  is  to  say,  holding  each  other  in  check 
by  their  reciprocal  action  :  such  substances  are  iron,  steel,  nickel, 
and  cobalt.  Magnets  also  contain  the  two  fluids,  but  there  is 
between  them  and  magnetic  substances  this  difference  :  that  in 
magnets  the  two  fluids  present  in  each  molecule  are  separated,  and 
each  produces  a  separate  effect ;  while  in  magnetic  substances  the 
fluids  are  combined  and  produce  no  effect. 

A  magnetic  substance  is  readily  distinguished  from  a  magnet. 
The  former  has  no  poles  ;  if  successively  presented  to  the  two  ends 
of  a  magnetic  needle,  ab  (fig.  322),  it  will  attract  both  ends  equally, 
while  one  end  of  a  magnet  would  attract  the  one,  but  repel  the 
other  end  of  the  needle.  Magnetic  substances  also  have  no  action 
on  each  other,  while  magnets  attract  or  repel,  according  as  unlike 
or  like  poles  are  presented  to  each  other. 

When  a  magnetic  substance  is  placed  in  contact  with  the  poles 
of  a  magnet,  the  north 
pole,  for  instance,  this 
acting  attractively  on  the 
south  fluid  of  the  sub- 
stance, and  repelling  the 
north  fluid,  it  follows  that 
in  this  body  a  separation 
of  the  two  fluids  is  effected, 
or,  in  other  words,  a  true 
magnet  is  produced.  For, 
if  any  piece  of  soft  iron, 

.  .          r  Fig.  323. 

an  iron  ring,  for  example, 

be  presented  to  a  magnetised  bar,  not  merely  is  this  ring  supported, 
but  it  acquires  the  property  of  supporting  the  second  ;  then  this 
second  a  third,  and  so  forth.  Remove  the  bar,  and  the  invisible  link 
which  unites  this  marvellous  chain  is  broken,  and  the  rings  separate. 
This  action  in  virtue  of  which  a  magnet  can  develope  magnetism 
in  iron,  is  called  magnetic  induction  or  influence,  and  it  can  take 
place  without  actual  contact  between  the  magnet  and  the  iron,  as 
is  seen  in  the  following  experiment.  A  bar  of  soft  iron  is  held  with 
one  end  near  a  magnetic  needle.  If  now  the  north  pole  of  a 
magnet  be  approached  to  the  other  end  of  the  iron  bar  without 


406  On  Magnetism.  [391- 

touching  it,  the  needle  will  be  attracted  or  repelled,  according  as 
its  south  or  north  pole  is  near  the  bar.  For  the  north  pole  ot 
the  magnet  will  develope  south  magnetism  in  the  end  of  the  bar 
nearest  it,  and,  therefore,  north  magnetism  at  the  other  end,  which 
would  thus  attract  the  south,  but  repel  the  north  end  of  the 
needle.  Obviously,  if  the  other  end  of  the  magnet  were  brought 
near  the  iron,  the  opposite  effects  would  be  produced  on  the  needle. 

Magnetic  induction  explains  the  formation  of  the  tufts  of  iron 
filings  which  become  attached  to  the  poles  of  magnets.  The  parts 
in  contact  with  the  magnet  are  converted  into  magnets  ;  these  act 
inductively  on  the  adjacent  parts,  these  again  on  the  following  ones, 
and  so  on,  producing  a  filamentary  arrangement  of  the  filings. 

392.  Coercive  force. — We  have  seen  from  the  above  experi- 
ments, that  soft  iron  becomes  instantaneously  magnetised  under  the 
influence  of  a  magnet,  but  that  this  magnetism  is  not  permanent, 
and  ceases  when  the  magnet  is  removed.  Steel  likewise  becomes 
magnetised  by  contact  with  a  magnet,  but  the  operation  is  effected 
with  difficulty,  and  the  more  so  as  the  steel  is  more  highly  tem- 
pered. Placed  in  contact  with  a  magnet,  a  steel  bar  acquires 
magnetic  properties  very  slowly,  and,  to  make  the  magnetism 
complete,  the  steel  must  be  rubbed  with  one  of  the  poles.  But 
this  magnetism,  once  evoked  in  steel,  is  permanent,  and  does  not 
disappear  when  the  inducing  force  is  removed. 

These  different  effects  in  soft  iron  and  steel  are  ascribed  to  a 
coercive  force,  which,  in  a  magnetic  substance,  offers  a  resistance 
to  the  separation  of  the  two  fluids,  but  which  also  prevents  their 
recombination  when  once  separated.  In  steel  this  coercive  force 
is  very  great,  in  soft  iron  it  is  very  small  or  even  quite  absent. 
By  oxidation,  pressure,  or  torsion,  a  certain  amount  of  coercive 
force  may  be  imparted  to  soft  iron,  as  will  be  explained  under 
Magnetisation. 


CHAPTER   II. 

TERRESTRIAL  MAGNETISM.      COMPASSES. 

393.  Directive  action  of  the  earth  on  magnets. — The  power 
of  attracting  iron  filings  is  not  the  only  one  which  magnets  present  ; 
they  have  also  that  of  setting  in  a  certain  definite  direction,  when 
they  can  turn  freely  in  a  horizontal  direction.  Thus,  when  a 


-394]  Magnetic  Meridian.     Declination. 


407 


magnetised  needle  is  placed  on  a  pivot  on  which  it  can  move  freely 
(fig.  324),  it  ultimately  sets  in  a  position  which  is  more  or  less  north 
and  south.  If  removed  from  this  position,  it  always  returns  to  it 
after  a  certain  number  of  oscillations. 

If,  instead  of  placing  the  needle  on  a  pivot,  it  be  placed  on  a 
cork,  and  this  in  turn  be  floated  on  water,  the  needle  will  after  a 
few  oscillations  come  into  a 
position  which  is  the  same  as 
that  it  had  on  the  pivot,  that 
is,  nearly  due  north  and  south. 
It  is  important  in  this  second 
experiment  to  observe,  that  the 
needle  only  sets  in  a  certain 
direction,  and  that,  though  free 
to  make  either  a  progressive 
or  a  retrograde  motion,  it  re- 
mains in  the  middle  of  the  vessel, 
and  moves  neither  towards  the 
north  nor  the  south ;  hence 
the  force  which  acts  upon  the 
middle  is  simply  directive,  and 
not  attractive. 

Analogous  observations  have 


Fig.  324. 


been  made  in  different  parts  of  the  globe,  from  which  the  earth 
has  been  compared  to  an  immense  magnet,  whose  poles  are  very 
near  the  terrestrial  poles,  and  whose  neutral  line  virtually  coincides 
with  the  equator. 

The  polarity  in  the  northern  hemisphere  is  called  the  northern 
or  boreal  polarity,  and  that  in  the  southern  hemisphere  the  southern 
or  austral  polarity.  In  French  works  the  end  of  the  needle  point- 
ing north  is  called  the  austral  or  southern  pole,  and  that  pointing 
to  the  south  the  boreal  or  northern  pole  ;  a  designation  based  on 
this  hypothesis  of  a  terrestrial  magnet,  and  on  the  law  that  unlike 
magnetisms  attract  each  other.  In  practice  it  will  be  found  more 
convenient  to  use  the  English  names,  and  call  that  end  of  the 
magnet  which  points  to  the  north  the  north  pole,  and  that  which 
points  to  the  south  the  south  pole. 

394.  Magnetic  meridian.  Declination. — When  a  magnetic 
needle  points  towards  the  north,  if  we  conceive  an  infinitely  long 
straight  line  passing  through  its  two  poles,  this  line  is  what  is 
called  the  magnetic  meridian  of  the  place.  The  direction  of  this 


408  On  Magnetism.  [394- 

line  does  not  in  general  coincide  with  the  geographical  meridian 
of  the  place,  which  is  the  imaginary  plane  which  passes  through 
this  place  and  through  the  earth's  poles.  The  angle  which  the 
direction  of  the  magnetic  needle  makes  with  the  geographical 
meridian  is  called  the  declination  of  the  place.  In  other  words, 
as  the  magnet  needle  does  not  exactly  point  to  the  earth's  nortfy 
the  declination  is  the  difference  between  this  direction  and  the 
true  north.  Sometimes  the  north  pole  of  the  needle  is  to  the  west 
of  the  meridian,  and  sometimes  it  is  to  the  east.  In  the  former 
case  the  declination  is  said  to  be  easterly ',  and  in  the  latter  case 
westerly. , 

The  declination  of  the  magnetic  needle,  which  varies  in  different 
places,  is  at  present  west  in  Europe  and  in  Africa,  but  east  in 
Asia  and  in  North  and  South  America.  It  shows  further  con- 
siderable variations  even  in  one  and  the  same  place. 

Thus,  at  London  the  needle  showed  in  1 580  an  east  declination 
of  11°  36'  ;  in  1663  it  was  at  zero;  from  that  time  it  gradually 
tended  towards  the  west,  and  reaching  its  maximum  declination 
of  24°  41 '  in  1818;  since  then  it  has  steadily  diminished  ;  it  was 
22°  30'  in  1850,  and  is  now  (1875)  19°  16'  W. 

At  Yarmouth  and  Dover  the  variation  is  about  40'  less  than  at 
London  ;  at  Hull  and  Southampton  about  20'  greater ;  at  New- 
castle and  Swansea  about  i°  15',  and  at  Liverpool  i°  30',  at  Edin- 
burgh 2°  5',  and  at  Glasgow  and  Dublin  about  2°  25',  greater  than 
at  London. 

Besides  these  variations,  which  are  called  secular  variations,  the 
declination  of  the  magnetic  needle  undergoes  accidental  variations, 
known  as  perturbations  or  magnetic  storms  ;  these  are  manifested 
during  the  occurrence  of  thunder  storms,  of  volcanic  eruptions,- 
and  during  the  appearance  of  aurora  borealis.  The  effect  of  the 
aurora  is  felt  at  great  distances.  Auroras  which  are  only  visible 
in  the  north  of  Europe  act  on  the  needle  even  in  these  latitudes. 
In  polar  regions  the  needle  frequently  oscillates  several  degrees  ;  its 
irregularity  on  the  day  before  the  aurora  borealis  is  a  presage  of  the 
occurrence  of  this  phenomenon. 

395.  mariner's  compass. — The  magnetic  action  of  the  earth 
has  received  a  most  important  application  in  the  mariner's  compass. 
This  is  a  declination  compass  used  in  guiding  the  course  of  a  ship. 
Fig.  325  represents  it  in  about  half  its  ordinary  size.  At  the 
bottom  of  a  wood  or  metal  box  is  drawn  a  star  or  rose,  with  sixteen 
branches,  representing  the  points  of  the  compass.  On  the  contour 


-395] 


Mariner  s  Compass. 


409 


of  the  box  is  a  graduated  circle,  the  zero  of  which  is  on  the  line 
NS,  which  marks  the  direction  from  north  to  south.  In  the  centre 
of  the  box,  finally,  is  a  steel  pivot,  on  which  rests  a  very  mobile 
magnetic  needle. 

When  the  geographical  meridian  is  known,  the  declination  is 
easily  determined  by  means  of  the  compass.  It  need  only  be 
turned  until  the  line  NS  is  exactly  in  the  direction  of  the  geographical 
meridian  of  the  place.  The  point  at  which  the  needle  stops  marks 
the  declination.  If,  on  the  contrary,  the  declination  is  known,  and 


Fig.  325. 

the  geographical  meridian  is  desired,  the  compass  is  turned  until  the 
needle  deviates  from  the  line  NS  by  a  quantity  equal  to  the  decli- 
nation, and  in  the  same  direction,  that  is  to  say,  to  the  east  if 
the  declination  is  easterly,  and  to  the  west  if  it  is  westerly ;  the 
line  NS  prolonged  represents  the  direction  of  the  geographical 
meridian. 

Neither  the  inventor  of  the  compass,  nor  the  exact  time  of  its 
invention,    is   known.     Guyot   de  Provins,  a  French   poet  of  the: 
twelfth  century,  first  mentions  the  use  of  the  magnet  in  navigation, 
though  it  is  probable  that  the  Chinese  long  before  this  had  used 
it.     The  ancient  navigators,  who  were  unacquainted  with  the  com-. 


4io 


On  Magnetism. 


[395- 


pass,  had  only  the  sun  or  pole-star  as  a  guide,  and  were  accord- 
ingly compelled  to  keep  constantly  in  sight  of  land,  for  fear  of 
steering  in  a  wrong  direction  when  the  sky  was  clouded.  But 
guided  by  the  indications  of  the  compass,  which  are  disturbed 
neither  by  darkness  nor  by  the  most  violent  tempests,  they  can 
pursue  their  true  route  by  night  as  well  as  by  day. 

The  compass  is  also  used  by  miners  to  direct  them  in  the  ex- 
cavation of  subterranean  passages. 

396.  Inclination  compass. — When  a  steel  needle  supported  on 
a  vertical  pivot,  as  represented  in  fig. 
324,  has  been  so  accurately  balanced 
that  it  is  quite  horizontal  before  magne- 
tisation, it  is  observed  that  when  it  is 
magnetised  it  ceases  to  retain  its  hori- 
zontal position,  and  the  north  pole  dips 
downward.  When  this  phenomenon 
was  first  observed,  it  was  ascribed  to  a 
defect  of  construction,  but  the  regu- 
larity with  which  it  occurred  proved 
that  it  must  be  ascribed  to  the  directive 
action  of  the  earth. 

In  order  to  observe  this  phenome- 
non, the  mode  of  suspension  is  modi- 
fied, and  it  is  fixed  to  a  horizontal 
axis,  so  that  it  can  move  in  a  vertical 
plane,  as  represented  in  fig.  326.  The 
angle  it  forms  is  read  off  on  the  divided 
circle. 

Fig.  326.  Thus   arranged,   the   apparatus   is 

called  the  declination  compass,  or  dipping  needle,  and  the  angle 
which  the  needle  makes  with  the  horizon  is  called  the  magnetic  in- 
clination QI  dip. 

The  value  of  the  dip,  like  that  of  the  declination,  differs  in  dif- 
ferent localities.  It  is  greatest  in  the  polar  regions,  and  decreases 
with  the  latitude  towards  the  equator,  where  there  is  a  series  of 
points  at  which  it  is  zero  ;  that  is,  at  which  the  needle  is  horizontal. 
The  line  joining  these  parts  is  called  the  terrestrial  magnetic  equa- 
tor. In  Greenwich  at  the  present  time  (1875)  the  dip  is  67°  42', 
reckoning  from  the  horizontal  line.  In  the  southern  hemisphere  the 
inclination  is  again  seen,  but  in  a  contrary  direction  ;  that  is,  the 
south  pole  of  the  needle  dips  below  the  horizontal  line. 


-397]  Methods  of  Magnetisation.  411 

The  terrestrial  magnetic  poles  are  those  places  in  which  the 
dipping  needle  stands  vertical ;  that  is,  where  the  inclination  is  90°. 
In  1830  the  first  of  these,  the  terrestrial  north  pole,  was  found  by 
Sir  James  Ross,  in  96°  43'  west  longitude  and  70°  north  latitude. 
The  same  observer  found  in  the  South  Sea,  in  76°  south  latitude  and 
i68°east  longitude,  that  the  inclination  was  88°  37'.  From  this  and 
other  observations,  it  has  been  calculated  that  the  position  of  the 
magnetic  south  pole  was  at  that  time  in  about  1 54°  east  longitude 
and  75^°  south  latitude. 

Lines  connecting  places  in  which  the  dipping  needle  makes  equal 
angles  are  called  isoclinic  lines. 

The  inclination  is  subject  to  secular  variations,  like  the  declina- 
tion. At  Paris,  in  1671,  the  inclination  was  75°;  since  then  it  has 
been  continually  decreasing  and  in  1859  was  66°  14'.  In  London 
also  the  dip  has  continually  diminished  since  1720  by  about  2*6' 
per  annum.  In  1821  it  was  70°  3' ;  in  1838,  69°  17';  in  1854  it 
was  68°  31' ;  in  1859  it  was  68°  21' ;  it  is  now  67°  42'.  It  is  also 
subject  to  slight  annual  and  diurnal  variations  ;  being,  according  to 
Hansteen,  about  15'  greater  in  summer  than  in  winter,  and  4'  or  5' 
greater  before  noon  than  after. 


CHAPTER    III. 

METHODS  OF   MAGNETISATION. 

397.  Magnetisation  by  the  influence  of  the  earth. — To  mag- 
netise a  substance  is  to  impart  to  it  the  magnetic  properties  of  attract- 
ing particles  of  iron,  and  of  turning  towards  the  north.  Magneti- 
sation can  be  produced  slowly  by  the  influence  of  the  earth,  or  more 
rapidly  by  rubbing  with  a  magnet ;  or  by  means  of  electricity,  in 
which  case  the  magnetisation  is  almost  instantaneous. 

The  magnetic  action  of  the  globe  is  powerful  enough  to  act  as  a 
source  of  magnetisation.  This  may  be  illustrated  by  taking  a 
tolerably  thick  iron  wire,  and  placing  it  in  the  magnetic  meridian, 
so  that  it  makes  an  angle  equal  to  the  angle  of  dip.  In  this  posi- 
tion the  earth's  magnetism,  acting  by  induction  on  the  iron  wire, 
decomposes  the  two  fluids,  and  converts  the  lower  end  into  a 
north  pole,  and  the  upper  into  a  south  pole.  Yet  this  magnetisa- 


412 


On  Magnetism. 


[397- 


tion  is  very  unstable,  for  if  the  wire  be  turned  upside  down,  the 
poles  are  inverted,  for  pure  soft  iron  is  destitute  of  coercive  force. 
But,  if  while  the  bar  is  in  the  above  position,  it  be  hammered,  or 
twisted,  the  pressure  or  the  twisting  imparts  to  it  a  certain 
amount  of  coercive  force,  and  it  retains  for  some  time  the  mag- 
netisation evoked  thus.  If  several  wires  thus  magnetised  are 
united  so  that  poles  of  the  same  name  are  together,  a  tolerably 
powerful  magnet  is  obtained. 

It  is  this  magnetising  action  of  the  earth  which  developes  the 
magnetism  frequently  observed  in  steel  and  iron  instruments,  such 
as  fire-irons,  railings,  lightning  conductors,  lamp  posts,  etc.,  which 
remain  for  some  time  in  a  more  or  less  inclined  position.  They 


Fig.  327. 

become  magnetised  with  the  north  pole  downward,  just  as  if  placed 
over  the  pole  of  a  powerful  magnet.  The  magnetism  of  native  black 
oxide  of  iron  has  doubtless  been  produced  by  the  same  causes  ; 
the  very  different  magnetic  power  of  different  specimens  being 
partly  attributable  to  the  different  positions  of  the  veins  of  ore 
with  regard  to  the  line  of  dip.  The  ordinary  irons  of  commerce 
are  not  quite  pure,  and  possess  a  feeble  coercive  force  ;  hence  a 
feeble  magnetic  polarity  is  generally  found  to  be  possessed  by  the 
tools  in  a  smith's  shop.  Cast-iron,  too,  has  usually  a  great  coercive 
force,  and  can  be  permanently  magnetised. 

The  turnings,  too,  of  wrought  iron  and  of  steel  produced  by  the 
powerful  lathes  of  our  .ironworks  are  found  to  be  magnetised. 


-397]  Magnetisation  by  Double  Touch.  413 

Magnetisation  by  magnets.  In  magnetising  bar  magnets,  and 
especially  magnetic  needles,  the  method  generally  adopted  is 
to  rub  them  with  powerful  magnets.  This  principle  is  applied 
in  the  methods  of  what  are  called  single,  separate,  and  double 
touch. 

The  method  of  single  touch  consists  in  moving  the  pole  of  a 
powerful  magnet  from  one  end  to  the  other  of  the  bar  to  be  mag- 
netised, and  repeating  this  operation  several  times,  always  in  the 
same  direction.  The  neutral  fluid  is  thus  gradually  decomposed 
throughout  all  the  length  of  the  bar,  and  that  end  of  the  bar  which 


Fig.  328. 


was  touched  last  by  the  magnet  is  ot  opposite  polarity  to  the  end 
of  the  magnet  by  which  it  has  been  touched.  This  method  only 
produces  a  feeble  magnetic  power,  and  is,  accordingly,  only  used 
for  small  magnets.  It  has  further  the  disadvantage  of  frequently 
developing  consequent  points. 

In  the  method  of  separate  touch  the  steel  bar  is  rubbed  sepa- 
rately with  the  contrary  poles  of  two  magnets,  proceeding  in  opposite 
directions  from  the  centre  towards  the  ends. 

Magnetisation  by  double  to^tch.  In  this  method  the  two  magnets 
are  placed  with  their  poles  opposite  each  other  in  the  middle  of  the 
bar  to  be  magnetised.  But,  instead  of  moving  them  in  opposite 


On  Magnetism. 


[397- 


directions  towards  the  two  ends,  as  in  the  method  of  separate 
touch,  they  are  kept  at  a  fixed  distance  by  means  of  a  piece  of  wood 
placed  between  them  (fig.  328),  and  are  simultaneously  moved  first 
towards  one  end,  then  from  this  to  the  other  end,  repeating  this 

operation  several  times,  and  finishing 
in  the  middle,  taking  care  that  each 
half  of  the  bar  receives  the  same 
number  of  frictions. 

Magnetisation  by  means  of  electri- 
cal currents  (476)  is  the  most  powerful 
means  of  imparting  magnetism,  and  is 
the  one  generally  used  for  large  mag- 
nets, whether  bar  or  horse-shoe. 

398.  Magnetic  batteries.  Arma- 
tures.— Magnetic  battery,  or  magazine, 
is  the  name  given  to  a  system  of  bars 
joined  with  their  similar  poles  together. 
Sometimes  the  bars  are  straight,  as  re- 
presented in  figs.  327  and  328,  and 
sometimes  they  are  curved,  as  in  fig. 
329,  which  represents  a  horse-shoe 
battery. 

Magnets,  whether  natural  or  arti- 
ficial, would  soon  lose  their  power  if 
they  were  left  to  themselves,  and  they 
must  therefore  be  provided  with 
armatures.  These  names  are  given  to 
pieces  of  soft  iron  which  are  placed 
in  contact  with  the  poles,  such  as  the 
piece,  ab,  in  'fig.  329.  The  two  poles 
Fis-  329-  of  the  magnet  acting  inductively  on 

this  piece  produce  in  it  at  a  a  north  pole,  and  at  b  a  south  pole,  and 
these  two  poles  thus  produced  react  in  turn  upon  the  magnetised 
bar,  and  by  preventing  the  recomposition  of  its  two  fluids  cause  it 
to  retain  its  force.  The  piece,  ab,  is  also  called  the  keeper  ;  to  it 
is  suspended  the  weights  which  the  magnet  is  intended  to  support. 


-399]  Frictional  Electricity.  415 


BOOK  VIII. 

FRICTIONAL   ELECTRICITY.         , 

CHAPTER    I. 
FUNDAMENTAL  PRINCIPLES. 

399.  Electricity.  Its  nature. — Electricity  is  a  powerful  physi- 
cal agent  which  manifests  itself  mainly  by  attractions  and  repulsions, 
but  also  by  luminous  and  heating  effects,  by  violent  shocks,  by  che- 
mical decompositions,  and  many  other  phenomena.  Unlike  gravity, 
it  is  not  inherent  in  bodies,  but  is  evoked  in  them  by  a  variety  of 
causes,  among  which  are  friction,  pressure,  chemical  action,  heat, 
and  magnetism. 

Thales,  one  of  the  Greek  sages,  600  B.C.,  knew  that  when  amber 
was  rubbed  with  silk  it  acquired  the  property  of  attracting  light 
bodies,  such  as  feathers,  pieces  of  straw,  etc.,  and  from  the  Greek 
form  of  this  word  (/jXeKrpov,  electron)  the  term  electricity  has  been 
derived.  Six  centuries  after  it  was  found,  Pliny,  the  celebrated 
Roman  naturalist,  writes,  '  When  the  friction  of  the  fingers  has  im- 
parted to  it  heat  and  life,  it  attracts  pieces  of  straw  as  a  magnet 
attracts  particles  of  iron/  This  is  nearly  all  the  knowledge  left  by 
the  ancients  ;  and  it  was  not  until  towards  the  end  of  the  sixteenth 
century  that  Dr.  Gilbert,  physician  .to  Queen  Elizabeth,  called 
attention  to  this  property  of  amber,  but  showed  also  that  it  was  not 
limited  to  amber,  but  that  other  bodies,  such  as  sulphur,  wax,  glass, 
etc.,  also  acquired  the  property  of  attracting  light  bodies  when  they 
are  rubbed  or  struck  with  flannel  or  with  catskin. 

To  repeat  this  experiment,  a  glass  rod,  or  a  stick  of  sealing  wax, 
or  shellac,  is  held  in  the  hand,  and  rubbed  with  a  piece  of  flannel, 
or  with  the  skin  of  a  cat  ;  it  is  then  found  that  the  parts  rubbed  have 
the  property  of  attracting  light  bodies,  such  as  pieces  of  silk,  wool, 


Frictional  Electricity. 


[399- 


feathers,  paper,  bran,  gold  leaf,  etc.,  which,  after  remaining  a  short 
time  in  contact,  are  again  repelled.  Not  only  have  the  substances 
thus  rubbed  the  property  of  attracting  light  particles,  but  they  also  be- 
come luminous  in  the  dark;  they  give  sparks,  and  presenta  number  of 

phenomena,  the  cause 
of  which  is  described 
under  the  general  term 
electricity,  the  deriva- 
tion of  which  has 
already  been  given. 

However  slow  the 
progress  of  the  science 
of  electricity  in  ancient 
times  and  in  the 
middle  ages,  its  pro- 
gress during  the  eigh- 
teenth and  nineteenth 
centuries  has  been  extremely  rapid.  In  the  last  seventy  or  eighty 
years,  more  especially,  the  new  facts  discovered  have  been  so 
numerous  and  remarkable,  their  applications  so  curious  and  impor- 
tant, that  electricity  has  been  compared  to  a  kind  of  fairy,  of  whom 
it  was  only  necessary  to  ask  miracles,  to  have  them  realised. 

400.  Sources  of  electricity. — The  causes  which  develope  elec- 
tricity are  numerous  ;  they  may  be  divided  into  mechanical,  physical, 
and  chemical  sources. 

The  mechanical  sources  are  friction,  pressure,  and  cleavage.  Thus, 
when  a  piece  of  sugar  is  broken  in  the  dark,  a  feeble  luminosity  is 
seen,  due  to  the  electricity  liberated.  Cleavage  is  also  a  source  of 
electricity  ;  if  a  plate  of  mica  be  rapidly  split  in  the  dark,  a  slight 
phosphorescence  is  perceived. 

The  physical  sources  are  variations  in  temperature  :  these  effects 
are  observed  in  some  minerals,  and  more  especially  in  tourmaline, 
which  exhibits  electrical  properties  when  either  heated  or  cooled. 

Lastly,  the  chemical  sources  are  the  combinations  and  the  de- 
compositions of  bodies.  Thus,  the  metals,  like  zinc,  iron,  copper, 
when  placed  in  an  acid,  unite  with  this  to  form  salts.  Now  during 
these  combinations  considerable  quantities  of  electricity  are  deve- 
loped ;  the  same  is  the  case  with  chemical  decomposition  ;  that  is, 
when  compound  bodies  are  separated  into  their  elements. 

The  most  powerful  sources  of  electricity  are  friction  and  chemi- 
cal action.  We  shall  first  of  all  investigate  the  influence  of  the  first 


-402] 


Distinction  of  Electricity. 


417 


cause;    and  shall  subsequently  investigate  the   latter  under  the 
name  of  VOLTAIC  ELECTRICITY. 

401.  Electroscopes.  Electrical  pendulum. — In  order  to  ascer- 
tain whether  bodies  are  electrified  or  not,  instruments  called  electro- 
scopes are  used.  The  simplest  of  these,  the  electric  pendulum 
(fig.  331),  consists  of  a  small  pith  ball  attached  by  means  of  a  silk 
thread  to  a  brass  rod  resting  on  a  glass  support.  To  ascertain 
whether  a  body  is  electrified  or  not,  it  need  only  be  presented  to  an 
electrical  pendulum  ;  in  the  first  case  there  is  attraction,  while  in 
the  second  case  there  is  not.  Yet  the  electrical  pendulum  would 
not  be  affected  by  a  body  very  feebly  charged  with  electricity- 


Fig.  331.  Fig.  332. 

More  complicated  and  more  delicate  apparatus  must  then  be  had 
recourse  to,  which  will  afterwards  be  described  (415,  430). 

402.  Distinction  of  the  two  kinds  of  electricity. — If  electri- 
city be  developed  on  a  glass  rod'  by  friction  with  silk,  and  the  rod 
be  brought  near  an  electrical  pendulum  (fig.  331),  the  ball  will  be  at- 
tracted to  the  glass,  and  after  momentary  contact  will  be  again 
repelled.  By  this  contact  the  ball  becomes  electrified,  and  so  long 
as  the  two  bodies  retain  their  electricity,  repulsion  follows  when 
they  are  brought  near  each  other.  If  a  stick  of  sealing  wax,  elec- 
trified by  friction  with  flannel  or  skin,  be  approached  to  another 
electrical  pendulum,  the  same  effects  will  be  produced,  the  ball  will 

E  E 


41 8  Frictional  Electricity.  [402- 

fly  towards  the  wax,  and  after  contact  will  be  repelled.  Two  bodies, 
which  have  been  charged  with  the  same  electricity,  repel  one  another. 
But  the  electricities,  respectively  developed  in  the  preceding  cases, 
are  not  the  same.  If,  after  the  pith  ball  has  been  touched  with  an 
electrified  glass  rod,  an  electrified  stick  of  sealing  wax,  and  then 
an  electrified  glass  rod,  be  alternately  approached  to  it.  the  pith 
ball  will  be  attracted  by  the  former  and  repelled  by  the  latter. 
Similarly,  if  the  pendulum  be  charged  by  contact  with  electrified 
sealing  wax,  it  will  be  repelled  vfaen.  this  is  approached  to  it,  but 
attracted  by  the  approach  of  the  electrically  excited  glass  rod. 

On  experiments  of  this  nature,  Dufay  first  made  the  observation 
that  there  are  two  different  electricities  :  the  one  developed  by 
the  friction  of  glass,  the  other  by  the  friction  of  resin  or  shellac. 
To  the  first  the  name  vitreous  electricity  is  given  ;  to  the  second 
the  name  resinous  electricity. 

403.  Hypothesis  of  two  electrical  fluids. — Notwithstanding 
the  great  importance  and  interest  of  the  numerous  electrical  pheno- 
mena we  are  still  ignorant  of  their  real  cause.  Various  hypotheses 
have  been  made  to  account  for  them.  The  most  convenient, 
perhaps,  is  that  which  was  propounded  by  Symmer,  an  English 
physicist. 

Symmer's  theory  assumes  that  every  body  contains  an  indefinite 
quantity  of  a  subtile  imponderable  matter,  which  is  called  the 
electrical  fluid.  This  fluid  is  formed  by  the  union  of  two  fluids — 
the  positive  and  the  negative.  When  they  are  combined  they 
neutralise  one  another,  and  the  body  is  then  in  the  natural  or 
neutral  state.  By  friction,  by  chemical  action,  and  by  several  other 
means,  this  neutral  fluid  may  be  decomposed  and  the  two  fluids- 
separated,  but  one  of  them  can  never  be  excited  without  a  simul- 
taneous production  of  the  other.  There  may,  however,  be  a  greater 
or  less  excess  of  the  one  or  the  other  in  any  body,  and  it  is  then  said 
to  be  electrified  positively  or  negatively.  The  two  fluids  were 
formerly  called  vitreous  and  resinous ,  but  these  have  given  place  to 
the  terms  positive  and  negative  fluids,  to  which  they  respectively 
correspond,  and  which  were  first  used  by  Franklin.  This  distinction 
is  merely  conventional ;  it  is  adopted  for  the  sake  of  convenience, 
and  there  is  no  other  reason  why  resinous  electricity  should  not 
be  called  positive  electricity. 

Fluids  of  the  same  name  repel  one  another,  and  fluids  of  opposite 
kinds  attract  each  other.  The  fluids  can  circulate  freely  on  the 
surface  of  certain  bodies,  which  are  called  conductors,  but  remain 


-405]  Coulomb's  Balance.  419 

confined  to  certain  parts  of  others,  which  are  called  nonconductors 
(406). 

As  has  been  already  said,  this  theory  is  quite  hypothetical ;  but 
its  general  adoption  is  justified  by  the  convenient  explanation  which 
it  gives  of  electrical  phenomena. 

404.  Laws  of  electrical  attraction  and  repulsion. — Adopting 
this  two-fluid  theory,  the  qualitative  and  quantitative  laws  of 
electrical  attraction  and  repulsion  may  be  stated  as  follows  : 

I.  Two  bodies  charged  with  the  same  electricity  repel  each  other ; 
two  bodies  charged  with  opposite  electricities  attract  each  other. 

II.  The  repulsions  or  attractions  between  two  electrified  bodies 
are  in  the  inverse  ratio  of  the  square  of  their  distances.     That  is  to 
say,  that  if  two  bodies  be  charged  to  a  certain  extent  with  elec- 
tricity, and  the  distance  between  them  be  increased  to  twice  or  thrice 
the  original  amount,  the  attraction  or  repulsion  will  be  one-fourth 
or  one-ninth  the  original  amount. 

III.  The  distance  remaining  the  same,  the  force,  of  attraction  or 
repulsion  between  two  electrified  bodies  is  directly  as  the  product  of 
the  quantities  of  electricity )  with  which  they  are  charged.     Thus  if 
the  quantity  of  electricity  with  which  a  body  is  charged  be  twice  or 
thrice  its  original  amount,  it  will  have  twice  or  three  times  the 
attractive  or  repulsive  force. 

These  attractions  and  repulsions  take  place  in  virtue  of  the  action 
which  the  two  electricities  exert  on  themselves,  and  not  in  virtue  of 
their  action  on  the  particles  of  matter. 

The  first  of  these  laws  follows  from  the  experiment  described 
above  (402)  ;  the  second  or  third  were  first  stated  by  Coulomb, 
and  may  be  demonstrated  by  an  apparatus  which  he  devised,  which 
is  known  as  Coulomb's  balance. 

405.  Coulomb's  balance.— Represented  in  fig.  333,  this  appa- 
ratus consists  of  a  cylindrical  glass  case  closed  at  the  top  by  a 
plate  of  the  same  material.  In  this  is  an  aperture  on  which  is 
a  glass  support,  d.  This  is  not  rigidly  fixed  but  can  be  turned 
round.  At  the  top  of  this  tube  is  a  brass  cap,  consisting  of  two 
pieces,  one  b,  which  is  rigidly  fixed  to  the  tube,  d,  and  the  other 
fitting  in  the  first  like  a  socket,  so  that  it  can  be  turned  by  the 
button,  /.  On  k  is  a  scale  e,  graduated  in  360  degrees,  and  turning 
with  it  ;  b  is  provided  with  a  fixed  index,  a,  which  shows  by  how 
many  degrees  the  disc  is  turned. 

To  the  disc  is  fixed  a  very  fine  silver  wire,  to  which  is  suspended 
a  shellac  thread,  p,  terminated  at  one  end  by  a  small  disc  of  thin 

E  E  2 


4-2  o 


Frictional  Electricity. 


[405- 


metal  foil,  n.  In  the  cover,  A,  near  the  edge,  is  a  second  aperture, 
through  which  can  be  passed  a  glass  rod,  z,  with  a  wooden  handle, 
r,  at  one  end,  and  terminating  in  a  brass  ball,  m.  A  scale  of  360 
degrees  is  fixed  on  the  cage,  opposite  the  zero  of  which  is  the 
ball,  m. 

In  experimenting  with  this  apparatus  the  air  is  dried  by  placing 
in  the  cage  some  chloride  of  calcium,  which  is  a  highly  hygroscopic 
substance.  To  establish  the  second  law,  that  electrical  attractions 
vary  inversely  as  the  square  of  the  distance,  the  disc,  e,  is  first 
turned  until  its  zero  corresponds  to  the  mark,  a  ;  the  tube,  d,  and 


Fig.  333- 

the  whole  cap,  k,  is  slowly  turned,  until  the  silver  thread  being- 
destitute  of  torsion,  and  the  needle,  p,  at  rest,  the  latter  corre- 
sponds to  the  zero  of  the  graduated  circle  :  the  knob,  ;«,  is  in  the 
same  position,  and  thus  presses  against  n.  The  knob,  m,  is  then 
removed  and  electrified,  and  replaced  in  the  apparatus,  through 
the  aperture,  r.  As  soon  as  the  electrified  knob,  m,  touches  «,  the 
latter  becomes  electrified,  and  is  repelled,  and  after  a  few  oscilla- 
tions comes  to  rest,  at  ten  degrees  for  instance  ;  the  resistance  of 
the  wire  to  further  torsion  then  just  balances  the  force  of  repulsion. 


-406]  Conductors  and  Nonconductors.  421 

As  the  arc  of  ten  degrees  is  virtually  the  same  as  its  chord,  the 
number  ten  may  be  regarded  as  representing  the  distance  of  m  and 
;/.  If  the  cap,  e,  is  turned  from  left  to  right  in  the  figure,  it  is  found 
that,  to  reduce  the  distance  to  five  degrees,  it  must  be  turned  through 
thirty-five  degrees.  The  wire  is  thus  twisted  through  thirty-five 
degrees  at  the  top  and  through  five  at  the  bottom  ;  its  total 
torsion  is  forty  degrees,  that  is  to  say,  four  times  as  much  as  it 
was  at  first.  Hence  at  the  distance  five  the  repulsion  is  four 
times  as  great  as  at  the  distance  ten  ;  for  it  is  a  known  law,  that 
the  angle  of  torsion  is  proportional  to  the  force  of  torsion.  It  may 
be  shown  in  the  same  manner  that,  to  make  the  distance  from  mto 
n  one-third  what  it  was,  the  total  torsion  must  be  ninety  degrees, 
that  is,  nine  times  as  great ;  the  second  law  is  thus  thereby  proved. 
In  order  to  prove  that  attractions  and  repulsions  between  electrified 
bodies  are  proportional  to  the  quantities  of  electricity  which  each 
of  them  possesses,  the  ball,  m,  is  again  electrified  and  placed  in  the 
cage  ;  after  contact  it  repels  the  disc,  n,  through  a  distance  of,  let  us 
say.  twelve  degrees.  The  ball,  m,  is  now  withdrawn  and  placed  in 
contact  with  a  second  brass  ball  of  the  same  diameter,  but  insulated 
and  unelectrified.  As  the  electricity  is  equally  distributed  over 
both  balls,  the  ball,  n,  loses  half  its  electricity,  and  on  again  placing 
it  in  the  cage,  the  repulsion  which  was  twelve  degrees  is  now  only 
six,  which  verifies  the  third  law. 

406.  Conductors  and  nonconductors. — When  a  glass  rod, 
rubbed  at  one  end,  is  brought  near  an  electroscope,  that  part  only 
will  be  electrified  which  has  been  rubbed  ;  the  other  end  will  pro- 
duce neither  attraction  nor  repulsion.  The  same  is  the  case  with 
a  rod  of  shellac  or  of  sealing  wax.  In  these  bodies  electricity  does 
not  pass  from  one  part  to  the  other — they  do  not  conduct  electricity. 
Experiment  shows,  that  when  a  metal  has  received  electricity  in 
any  of  its  parts,  the  electricity  instantly  spreads  throughout  its 
entire  surface.  Metals  are  hence  said  to  be  good  conductors  of 
electricity. 

Bodies  have,  accordingly,  been  divided  into  conductors  and  non- 
conductors. This  distinction  is  not  absolute,  and  we  may  advan- 
tageously consider  all  bodies  as  offering  a  resistance  to  the  passage 
of  electricity  which  varies  with  the  nature  of  the  substance.  Those 
bodies  which  offer  little  resistance  are  then  conductors,  and  those 
which  offer  great  resistance  are  nonconductors  or  insulators  :  elec- 
trical conductivity  is  thus  the  inverse  of  electrical  resistance.  We 
are  to  consider  that  between  conductors  and  nonconductors  there 


422  Frictional  Electricity.  [406- 

is  a  qitantitative  and  not  a  qualitative  difference  ;  there  is  no  con- 
ductor so  good  but  that  it  offers  some  resistance  to  the  passage  of 
electricity,  nor  is  there  any  substance  which  insulates  so  completely 
but  that  it  allows  some  electricity  to  pass.  The  transition  from 
conductors  to  nonconductors  is  gradual,  and  no  sharp  line  of  de- 
marcation can  be  drawn  between  them. 

In  this  sense  we  are  to  understand  the  following  table  in  which 
bodies  are  classed  as  conductors,  semiconductors,  and  noncon- 
ductors ;  those  bodies  being  conveniently  designated  as  conductors 
which,  when  applied  to  an  electroscope  charged  with  either  kind  of 
electricity  discharge  it  almost  instantaneously ;  semiconductors 
being  those  which  discharge  it  in  a  short  but  measurable  time,  a 
few  seconds,  for  instance  :  while  nonconductors  effect  no  discharge 
even,  in  the  course  of  a  minute. 

Conductors.       Semiconductors.  Nonconductors. 

Metals.  Alcohol  and  ether.         Dry  oxides. 

Graphite.          Powdered  glass.  Air  and  dry  gases. 

Acids.  Dry  wood.  Dry  paper. 

Water.  Silk. 

Snow.  Diamond  and  precious  stones. 

Vegetables.  .  Caoutchouc. 

Animals.  Glass. 

Sulphur. 

Resins. 

407.  Insulating  bodies.  Common  reservoir.  Electrification 
of  conductors. — Bad  conductors  are  called  insulators,  for  they  are 
used  as  supports  for  bodies  in  which  electricity  is  to  be  retained. 
A  conductor  remains  electrified  only  so  long  as  it  is  surrounded  by 
insulators.  If  this  were  not  the  case,  as  soon  as  the  electrified 
body  came  in  contact  with  the  earth,  which  is  a  good  conductor, 
the  electricity  would  pass  into  the  earth,  and  diffuse  itself  through 
its  whole  extent.  On  this  account,  the  earth  has  been  named  the 
common  reservoir.  A  body  is  insulated  by  being  placed  on  a 
support  with  glass  feet,  or  on  a  resinous  cake,  or  by  being  sus- 
pended by  silk  threads.  No  bodies,  however,  insulate  perfectly  ;  all 
electrified  bodies  lose  their  electricity  more  or  less  rapidly  by 
means  of  the  supports  on  which  they  rest.  Glass  is  in  itself  a  very 
perfect  insulator  but  it  is  always  somewhat  hygroscopic,  and  the 
aqueous  vapour  which  condenses  on  it  affords  a  passage  for  the 
electricity ;  the  insulating  power  of  glass  is  materially  improved  by 


-408]        Development  of  Electricity  by  Friction.  423 

coating  it  with  shellac  or  copal  varnish.  Dry  air  is  a  good  insulator  ; 
but,  when  the  air  contains  moisture,  it  conducts  electricity,  and  this 
is  the  principal  source  of  the  loss  of  electricity. 

It  is  from  their  great  conductivity,  that  metals  do  not  become 
electrified  by  friction.  But  if  they  are  insulated,  and  then  rubbed, 
they  give  good  indications.  This  may  be  seen  by  the  following 
experiment.  A  brass  tube  is  provided  with  a  glass  handle,  by 
which  it  is  held,  and  then  rubbed  with  silk  or  flannel.  On  ap- 
proaching the  metal  to  the  electric  pendulum  (fig.  331),  the  pith 
ball  will  be  attracted.  If  the  metal  is  held  in  the  hand  electricity  is 
indeed  produced  by  friction,  but  it  immediately  passes  through 
the  body  into  the  ground. 

Electrifying  by  contact  is  due  to  conductivity.  For  when  an 
insulated  conductor  in  the  neutral  state  is  made  to  touch  an  elec- 
trified conductor,  a  portion  of  the  latter  passes  instantaneously  to 
the  former.  If  the  two  bodies  have  the  same  surface,  and  the  same 
shape,  for  instance,  two  spheres  of  the  same  diameter,  the  electricity 
is  equally  distributed  on  the  two ;  but  if  the  bodies  differ  in  shape 
or  surface  the  electricity  is  unequally  distributed. 

408.  Xiaw  of  the  development  of  electricity  by  friction. — 
Whenever  two  bodies  are  rubbed  together,  the  neutral  fluid  is  de- 
composed. The  two  electricities  are  developed  at  the  same  time 
and  in  equal  quantities — one  body  lakes  the  positive,  and  the  other 
the  negative  fluid.  This  may  be  proved  by  the  following  simple 
experiment  devised  by  Faraday: — A  small  flannel  cap  provided 
with  a  silk  thread  is  fitted  on  the  end  of  a  stout  rod  of  shellac,  arid 
rubbed  round  a  few  times.  When  the  cap  is  removed  by  means  of 
a  silk  thread,  and  presented  to  a  pith  ball  pendulum  charged  with 
positive  electricity,  the  latter  will  be  repelled,  proving  that  the 
flannel  is  charged  with  positive  electricity  ;  while,  if  the  shellac  is 
presented  to  the  pith  ball,  it  will  be  attracted,  showing  that  the 
shellac  is  charged  with  negative  electricity.  Both  electricities  are 
present  in  equal  quantities  ;  for  if  the  rod  be  presented  to  the  elec- 
troscope before  removing  the  cap,  no  action  is  observed. 

The  electricity  developed  on  a  body  by  friction  depends  on  the 
body  rubbed.  Thus  glass  becomes  negatively  electrified  when 
rubbed  with  catskin,  but  positively  when  rubbed  with  silk.  In 
the  following  list  the  substances  are  arranged  in  such  an  order,  that 
each  becomes  positively  electrified  when  rubbed  with  any  of  the 
bodies  following,  but  negatively  when  rubbed  with  any  of  those 
which  precede  it  : 


424 


Frictional  Electricity. 


[408- 


1.  Catskin. 

2.  Flannel. 

3.  Glass. 

4.  Silk. 

5-  The  hand. 
6.  Wood. 


7.  Metals. 

8.  Caoutchouc. 

9.  Resin. 

10.  Sulphur. 

11.  Gutta  percha. 

12.  Gun-cotton. 


409.  Accumulation  of  electricity  on  the  surface  of  bodies. — 

Numerous  experiments  show  that  when  a  body  is  electrified,  all  the 
electrical  fluid  goes  to  the  surface,  where  it  is  accumulated  as  an 
extremely  thin  layer,  tending  incessantly  to  escape,  and  flying  off 
in  short,  when  it  is  not  retained  by  any  obstacle. 


Fig.  335- 

This  may  be  demonstrated  by  the  following  experiment,  which 
is  due  to  Biot. 

A  hollow  brass  globe,  fixed  to  an  insulating  support,  is  provided 
with  two  brass  hemispherical  envelopes  which  fit  closely,  and  can 
be  separated  by  glass  handles.  The  interior  is  now  electrified,  and 


-410]  Pozver  of  Points.  425 

the  two  hemispheres  brought  in  contact.  On  then  rapidly  removing 
them  (fig.  335),  the  coverings  will  be  found  to  be  electrified,  while 
the  sphere  is  in  its  natural  condition,  and  indicates  no  electricity. 
Thus  in  removing,  so  to  say,  the  surface  of  a  body,  all  the  free 
electricity  it  contained  is  also  removed,  which  shows  clearly  that 
the  electricity  is  on  the  surface.  That  electricity  resides  solely  in 
the  surface  is  further  proved  by  the  fact,  that  two  metal  spheres  of 
the  same  diameter,  but  one  of  them  solid  and  the  other  hollow, 
take  the  same  charge  of  electricity  when  applied  to  the  same 
source. 

The  same  point  may  also  be  illustrated  by  means  of  a  bird  cage, 
preferably  of  metal  wire,  which  is  suspended  by  insulators  and 
contains  either  a  gold  leaf  electroscope,  or  pieces  of  Dutch  metal, 
feathers,  etc.  When  the  cage  is  charged  by  being  connected  with  an 
electrical  machine  at  work,  the  articles  in  the  interior  are  quite  un- 
affected, although  strong  sparks  may  be  taken  from  the  outside.  A 
bird  in  the  inside  is  quite  unaffected  by  the  charge  or  discharge  of 
the  electricity  of  the  cage. 

When  accumulated  on  the  surface  of  bodies,  electricity  tends  to 
pass  off  to  adjacent  objects  with  an  effort  which  is  known  as  the 
tension.  This  increases  with  the  quantity  of  electricity.  So  long 
as  it  does  not  exceed  a  certain  limit,  it  is  balanced  by  the  resistance 
presented  by  the  small  conducting  power  of  the  air  when  it  is  dry. 
If  the  tension  increases,  this  resistance  is  overcome,  and  the  elec- 
tricity springs  off  to  an  adjacent  body  with  a  sound,  and  in  the  form 
of  a  bright  spark.  In  moist  air  the  tension  is  always  feeble,  for  the 
electricity  passes  away  almost  as  rapidly  as  it  is  supplied,  moisture 
being  a  good  conductor  of  electricity.  In  very  rarefied  air,  on  the 
contrary,  where  there  is  little  resistance,  electricity  passes  off, 
presenting  the  appearance  of  a  luminous  glow. 

410.  Influence  of  the  shape  of  a  body  on  the  accumulation 
of  electricity.  Power  of  points. — The  manner  in  which  electricity 
is  distributed  on  the  surface  of  a  body  varies  with  its  shape.  If  it 
is  spherical  the  amount  is  everywhere  the  same,  which  might  in- 
deed be  predicted,  and  which  may  be  readily  confirmed  by  means 
of  the  proof  plane.  This  is  a  small  thin  metal  disc  fixed  at  the  end 
of  a  thin  shellac  rod.  This  is  held  in  the  hand,  and  successively 
applied  to  different  parts  of  the  electrified  body,  and  after  each 
contact  is  presented  to  an  electrical  pendulum.  If  the  body  is  a 
sphere  the  attraction  is  in  each  case  the  same,  which  shows  that 
the  disc  has  taken  the  same  charge  of  electricity  from  each  part 


426 


Fractional  Electricity. 


[410- 


of  the  sphere,  and,  therefore,  that  the  distribution  of  the  electrical 
fluid  is  uniform. 

This  is  no  longer  the  case  if  the  electrified  body  is  more  or  less 
elongated,  as,  for  instance,  a  kind  ef  ovoid  shape,  as  shown  in  fig. 
336.  In  this  case  the  proof  plane  is  the  more  charged  the  nearer 
it  is  applied  to  the  elongated  end  ;  and  at  this  end  itself  most 
electricity  is  removed.  This  experiment  shows  that,  in  good  con- 
ductors, electricity  always  tends  to  accumulate  towards  the  most 


Fig.  336. 

elongated  parts,  towards  the  points.  This  accumulation  produces 
a  greater  tension,  which  is  sufficient  to  overcome  the  resistance  of 
the  air,  and  allow  electricity  to  escape.  It  is  in  fact  observed,  that 
metal  bodies  provided  with  a  point  quickly  lose  their  electricity, 
and,  if  the  hand  be  held  over  such  a  point,  a  sort  of  wind  or  draught 
is  felt.  If  this  takes  place  in  darkness,  a  kind  of  luminous  brush 
appears  on  the  top  of  the  point. 

This  property  of  points,  placed  on  electrified  conductors,  of  al- 
lowing electricity  to  escape,  has  been  called  the  power  of  points  ; 
and  in  electrical  experiments  we  meet  with  numerous  instances 
where  it  comes  into  play. 


-411] 


Electrical  Induction. 


427 


CHAPTER   II. 

ACTION   OF   ELECTRIFIED   BODIES  ON   BODIES   IN   THE   NATURAL 
STATE  ;   INDUCED    ELECTRICITY.      ELECTRICAL  MACHINES. 

411.  Electricity  by  influence  or  induction. — An  insulated 
conductor,  charged  with  either  kind  of  electricity,  acts  on  bodies 
in  a  natural  state  placed  near  it,  in  a  manner  analogous  to  that  of 
the  action  of  a  magnet  on  soft  iron,  that  is,  it  decomposes  the 
neutral  fluid,  attracting  the  opposite,  and  repelling  the  like  kind  of 
electricity.  The  action,  which  is  a  consequence  of  the  attractions 
and  repulsions  of  the  two  electricities,  and  which  is  exerted  not  only 
through  air  but  also  through  insulating  bodies  like  air,  glass,  resins, 
etc.,  is  said  to  take  place  by  influence  or  induction. 


The  phenomena  of  induction  may  be  demonstrated  by  means  of 
the  experiment  represented  in  fig.  337.  On  the  right  hand  of  the 
figure  is  the  conductor  of  the  electrical  machine,  which,  as  we  shall 
afterwards  see,  is  charged  with  positive  electricity  ;  on  the  left  is  a* 
brass  cylinder,  insulated  by  being  placed  on  a  glass  support,  and 
provided  with  small  pith  ball  pendulums,  suspended  by  linen 


428  Frictional  Electricity.  [411- 

threads,  which  are  conductors.  When  the  cylinder  of  the  machine 
is  brought  near  this  conductor,  the  pendulums  are  found  to  diverge 
but  to  unequal  extents,  the  greatest  divergence  being  met  with  at 
the  ends.  Near  the  middle  the  pith  balls  do  not  diverge  at  all ; 
the  electricity  is,  therefore,  accumulated  at  the  ends,  and  the 
middle  is  in  the  neutral  state.  If,  moreover,  a  sealing-wax  rod  which 
has  been  rubbed  with  flannel  be  approached  to  the  pendulums 
nearest  the  electrical  machine,  they  will  be  repelled,  showing  that 
they  are  charged  with  the  same  electricity  as  the  rubbed  sealing  wax, 
that  is,  with  negative  electricity.  If,  in  like  manner,  a  glass  rod, 
which  has  been  rubbed  with  silk,  be  approached  to  the  other  end  of 
the  cylinder,  the  pendulums  are  also  repelled,  which  shows  that  they 
are  charged  with  positive  electricity.  The  electricities  thus  sepa- 
rated are  equal  in  quantity,  for  if  the  machine  is  removed  all  the 
pendulums  cease  to  diverge,  since  the  two  electricities  have  recom- 
bined,  and  the  body  is  restored  to  the  neutral  state. 

This  electrifying  by  influence,  or  induction  as  it  is  called,  which 
is  produced  by  an  electrified  body  or  bodies  in  the  neutral  state 
explains  a  host  of  phenomena.  In  order  to  explain  all  its  effects, 
it  is  important  to  inquire  what  takes  place  when,  in  the  above  ex- 
periment, the  insulated  cylinder  is  placed  for  a  short  time  in  con- 
tact with  the  ground,  while  it  is  still  under  the  influence  of  the 
machine.  Suppose,  for  instance,  the  further  end  be  placed  in  con- 
tact with  the  ground,  the  positive  electricity  will  escape,  while  the 
negative  remains  held  by  the  attraction  of  the  opposite  electricity 
of  the  machine.  If  now  connection  with  the  ground  be  interrupted, 
and  the  cylinder  be  moved  away  from  the  influence  of  the  machine, 
the  pendulums  will  diverge,  and,  as  can  be  easily  verified,  owing  to 
their  being  charged  with  negative  electricity.  Even  if  the  end 
nearest  the  machine  be  connected  with  the  ground  the  result  is  still 
the  same.  The  negative  electricity  does  not  pass  into  the  ground  ; 
it  is  the  positive  which  still  escapes  ;  the  negative  being  attracted 
by  the  contrary  electricity  of  the  machine,  on  interrupting  the 
communication  with  the  earth,  the  cylinder  remains  charged  with 
negative  electricity. 

Thus  a  body  can  be  charged  with  electricity  by  induction  as  well 
as  by  conduction.  But,  in  the  latter  case,  the  charging  body  loses 
part  of  its  electricity,  which  remains  unchanged  in  the  former  case. 
The  electricity  imparted  by  conduction  is  of  the  same  kind  as  that 
of  the  electrified  body,  while  that  excited  by  induction  is  of  the 
opposite  kind.  To  impart  electricity  by  conduction,  the  body  must 


-412]  Ramsdeiis  Electrical  Machine.  429 

be  quite  insulated,  while,  in  the  case  of  induction,  it  must  be  in  con- 
nection with  the  earth,  at  all  events,  momentarily. 

What  has  here  been  said  has  referred  to  the  inductive  action  ex- 
erted on  good  conductors.  Bad  conductors  are  not  so  easily  acted 
upon  by  induction,  owing  to  the  great  resistance  they  present  to  the 
circulation  of  electricity,  but,  when  once  charged,  the  electric  state 
is  more  permanent. 

This  is  analogous  to  what  is  met  with  in  magnetism  ;  a  magnet 
instantaneously  evokes  magnetism  in  a  piece  of  soft  iron  ;  but  this 
is  only  temporary,  and  depends  on  the  continued  action  of  the 
magnet ;  a  magnet  magnetises  steel  with  far  greater  difficulty,  but 
this  magnetism  is  permanent. 

• 

ELECTRICAL  MACHINES. 

412.  Ramsden's  electrical  machine. —  The  first  electrical 
machine  was  invented  by  Otto  von  Guericke,  the  inventor  also  of 
the  air-pump.  It  consisted  of  a  sphere  of  sulphur,  which  was 
turned  on  an  axis  by  means  of  the  hand,  while  the  other,  pressing 
against  it,  served  as  a  rubber.  Resin  was  afterwards  substituted 
for  the  sulphur,  which  in  turn,  Hawksbee  replaced  by  a  glass 
cylinder.  In  all  these  cases  the  hand  served  as  rubber ;  and 
Winckler,  in  1740,  first  introduced  cushions  of  horsehair  covered 
with  silk  as  rubbers.  At  the  same  time,  Bose  collected  electricity 
disengaged  by  friction,  on  an  insulated  cylinder  of  tin  plate.  Lastly, 
Ramsden,  in  1 760,  replaced  the  glass  cylinder  by  a  circular  glass 
plate,  which  was  rubbed  by  cushions.  The  form  which  the  machine 
has  now  is  but  a  modification  of  Ramsden's  original  machine. 

Between  two  wooden  supports  (fig.  338),  a  circular  glass  plate, 
P,  about  a  yard  in  diameter,  is  suspended  by  an  axis  passing 
through  the  centre,  and  which  is  turned  by  means  of  a  glass  handle. 
The  plate  revolves  between  two  sets  of  cushions  or  rubbers,  of 
leather  or  of  silk,  one  set  above  the  axis  and  one  below,  which,  by 
means  of  screws,  can  be  pressed  as  tightly  against  the  glass  as  may 
be  desired,  by  which  means  'the  plate  becomes  electrified  on  both 
sides.  In  front  of  the  plate  also  are  two  brass  rods,  provided  with 
a  series  of  points  in  the  sides  opposite  the  glass  ;  these  rods  are  fixed 
to  two  large  metal  cylinders,  A  A,  which  form  the  prime  conductor. 
The  latter  are  insulated  by  being  supported  on  glass  feet,  and  are 
connected  with  each  other  by  a  smaller  rod. 

The  action  of  the  machine  is  founded  on  the  excitation  of  elec- 


430 


Frictional  Electricity. 


[412- 


tricity  by  friction,  and  on  the  action  of  induction.  By  friction  with 
the  rubbers,  the  glass  becomes  positively,  and  the  rubbers  nega- 
tively electrified.  If  now  the  rubbers  were  insulated,  they  would 
receive  a  certain  charge  of  negative  electricity  which  it  would  be 


Fig.  338. 

impossible  to  exceed,  for  the  tendency  of  the  opposed  electricities 
to  reunite  would  be  equal  to  the  power  of  the  friction  to  decom- 
pose the  neutral  fluid.  But  the  rubbers  communicate  with  the 
ground  by  means  of  bands  of  tinfoil,  fixed  to  the  supports,  not 
shown  in  the  figure,  and,  consequently,  as  fast  as  the  negative  elec- 
tricity is  generated  it  passes  off.  The  positive  electricity  of  the 
glass  acts  then  by  induction  on  the  conductor,  attracting  the  nega- 


-413]  Measurement  of  the  Charge.  43 1 

tive  fluid.  The  conductors  thus  lose  their  negative  electricity,  and 
remain  charged  with  positive  fluid.  The  plate  accordingly  gives  up 
nothing  to  the  conductors  ;  in  fact,  it  only  abstracts  from  them  their 
negative  fluid. 

As  thus  described,  the  electrical  machine  yields  only  positive 
electricity ;  it  may,  however,  be  arranged  so  as  to  give  negative 
electricity.  For  this  purpose  the  four  feet  of  the  table  are  insulated 
by  being  placed  on  thick  plates  of  resin,  of  glass,  or  of  sulphur,  and 
the  conductors  are  connected  with  the  ground  by  a  metallic  chain. 
This  allows  the  electricity  of  the  positive  conductors  to  escape, 
while  the  negative  electricity  of  the  rubbers  accumulates  on  the 
supports  and  on  the  bands  of  tinfoil. 

413.  Measurement  of  the  charge  of  the  electrical  machine. 
Quadrant  electrometer. — The  amount  of  electrical  charge,  or 
electric  tension,  is  measured  by  the  quadrant  or  Henley's  electro- 
meter, which  is  represented  in  fig.  339  attached  to  the  conductor. 
This  is  a  small  electric  pendulum,  consisting  of  a  wooden  rod, 
to  which  is  attached  an  ivory  or  cardboard  scale  (fig.  339).  In 
the  centre  of  this  is  a  small  whalebone  index,  movable  on  an  axis, 
and  terminating  in  a  pith  ball.  Being  attached  to  the  conductor, 
the  index  rises  as  the  machine  is  charged,  ceasing  to  rise  when  the 
limit  is  attained.  When  the  rotation  is  discontinued  the  index  falls 
rapidly  if  the  air  is  moist  ;  but  in  dry  air  it  only  falls  slowly,  show- 
ing, therefore,  that  the  loss  of  electricity  in  the  latter  case  is  less  • 
than  in  the  former. 

Hence  in  moist  and  rainy  weather  all  experiments  with  the  elec- 
trical machine  are  difficult  to  perform.  All  parts  of  the  apparatus 
must  be  carefully  warmed  by  a  charcoal  chauffer,  and  the  supports 
and  plate  must  be  rubbed  with  hot  cloths. 

The  rubbers  require  great  care  both  in  their  construction  and  in 
their  preservation.  They  are  commonly  made  of  leather  stuffed 
with  horse-hair.  Before  use  they  are  coated  either  with  powdered 
aurum  musivum  (sulphuret  of  tin),  or  graphite,  or  amalgam.  The 
action  of  these  substances  is  not  very  clearly  understood.  Some 
consider  that  it  merely  consists  in  promoting  friction.  Others 
again  believe  that  a  chemical  action  is  produced,  and  assign 
in  support  of  this  view  the  peculiar  smell  noticed  near  the  rubbers 
when  the  machine  is  worked.  Amalgams,  perhaps,  promote  most 
powerfully  the  disengagement  of  electricity.  Kienmayer's  amalgam 
is  the  best  of  them. 

Whatever  precautions  be  taken  to  avoid  the  loss  of  electricity, 


432  F fictional  Electricity.  [413- 

or  however  rapidly  the  machine  is  turned,  it  is  impossible  to  ex- 
ceed a  certain  limit.  For  as  the  electricity  accumulates  on  the 
machine,  its  tension  increases  too,  and  very  soon  its  tendency  to 
escape  exceeds  the  resistance  offered  by  the  air  and  the  supports  of 
the  conductors.  From  this  moment  the  loss  of  electricity  equals  the 
electricity  disengaged  by  friction,  and  hence  the  tension  can  never 
exceed  the  limit  it  has  attained,  which  is  indicated  by  the  electro- 
meter remaining  stationary  although  the  rotation  is  continued. 

If,  moreover,  the  maximum  effect  is  desired,  the  machine  must 
not  be  placed  too  near  the  walls  or  the  furniture  ;  in  short,  away 
from  all  objects  on  which  it  could  act  by  induction,  especially  if  these 


Fig-  339- 

are  angular,  for  it  then  continually  withdraws  the  negative  electricity 
and  tends  to  revert  to  the  neutral  state.  Thus  if  a  point  be  presented 
to  a  machine  in  action,  as  represented  in  fig.  339,  the  electrometer 
falls,  even  though  the  point  is  at  some  distance.  This  is  due  to  the 
fact  that  the  positive  electricity  of  the  machine  induces  negative  in 
the  point,  which  flows  out  as  fast  as  it  is  produced,  and  combining 
with  the  positive  by  means  of  which  it  was  evoked,  continually 
brings  the  machine  back  to  the  neutral  state. 

414.  Electrophorus. — This  is  a  very  simple  apparatus  invented 
by  Volta,  and  by  means  of  which  considerable  quantities  of  elec- 
tricity may  be  produced.  It  consists  of  a  cake  of  resin,  C  (fig.  340), 
of  about  twelve  inches  diameter,  and  an  inch  thick,  which  is  placed 


-414] 


Elcctrophorus. 


433 


on  a  metallic  surface,  or  very  frequently  fits  in  a  wooden  mould 
lined  with  tinfoil,  which  is  called  the  form.  Besides  this,  there  is  a 
wooden  disc,  of  a  diameter  somewhat  less  than  that  of  the  cake, 
lined  on  its  under  surface  with  tinfoil,  and  provided  with  an  insu- 
lating glass  handle.  This  is  called  the  cover.  The  mode  of 
working  this  apparatus  is  as  follows  :  All  the  parts  of  the  apparatus 
having  been  well  warmed,  the  cake,  which  is  placed  in  the  form,  or 
rests  on  a  metallic  surface,  is  briskly  flapped  with  a  catskin,  as 
shown  in  fig.  340,  by  which  it  becomes  charged  with  negative 


Fig.  340. 

electricity.  The  cover  held  by  the  insulating  handle  is  then  placed 
on  the  cake.  The  negative  electricity  of  the  cake  acting  thus  in- 
ductively on  the  cover  attracts  positive  electricity  to  the  lower  sur- 
face, and  repels  negative  to  the  upper.  If  now  this  upper  surface  be 
touched  by  the  finger,  as  shown  in  fig.  341,  the  negative  electricity 
passes  out  into  the  ground,  and  the  disc  only  retains  positive  elec- 
tricity. Now  when  the  cover  is  raised  by  one  hand  by  means  of  the 
insulating  handle,  and  the  other  hand  is  brought  near  it,  a  smart 

F  F 


434 


Frictional  Electricity. 


[414- 


spark  passes,  due  to  the  recombination  of  the  positive  of  the  disc 
with  the  negative  produced  by  its  induction  in  the  hand  (fig.  342). 

Replacing  the  disc  upon  the  cake,  this  again  exerts  its  inductive 
action,  for  it  is  such  a  bad  conductor  that  the  electricity  does  not 
pass  off  to  the  cover,  and  if  the  same  operations  be  repeated,  a 
succession  of  such  sparks  may  be  obtained  even  after  the  lapse  of 
some  time.  The  retention  of  electricity  is  greatly  promoted  by 
keeping  the  cake  in  the  form,  and  placing  the  cover  upon  it,  by 


Fig.  342. 


which  the  access  of  air  is  hindered.  Instead  of  a  cake  of  resin,  a 
disc  of  gutta  percha,  or  vulcanised  cloth,  or  vulcanite,  may  be  sub- 
stituted ;  and  of  course,  if  any  material  which  becomes  positively 
electrified  by  friction  be  used,  the  cover  acquires  a  negative  charge. 
415.  Gold  leaf  electroscope. — The  gold  leaf  electroscope,  also 
called  Sennet fs  electroscope,  from  the  name  of  its  inventor,  is  a  small 
but  delicate  apparatus  for  ascertaining  whether  a  body  is  electrified, 
and  if  so  with  what  kind  of  electricity  it  is  charged.  It  consists  of 
a  tubulated  glass  shade  (fig.  343),  the  neck  of  which  is  closed  by  a 
cork.  In  this  is  fitted  a  brass  rod  terminating  at  the  top  in  a  knob, 
and  at  the  bottom  in  two  strips  of  gold  leaf.  The  neck,  the  cork,. 


-415] 


Gold  Leaf  Electroscope. 


435 


and  the  upper  part  of  the  shade  are  coated  with  a  thick  layer  of 
sealing  wax  varnish,  which  is  nothing  more  than  a  solution  of  seal- 
ing wax  in  spirits  of  wine.  The  object  of  this  coating  is  to  improve 
the  insulating  qualities  of  the  glass.  Glass  is  indeed  a  bad  conduc- 
tor, but  it  is  very  hygroscopic  ;  that  is,  it  readily  attracts  aqueous 
vapour  from  the  air,  and  thus  becomes  coated  with  a  layer  of  moisture 
which  renders  its  surface  a  conductor.  When  covered  with  varnish 
this  evil  is  removed,  for  varnishes,  which  are  usually  made  of  resin, 
are  not  at  all  hygroscopic. 


Fig.  343- 

The  air  in  the  inside  is  dried  by  quicklime,  or  by  chloride  of 
calcium,  and  on  the  insides  of  the  shade  there  are  two  strips  of 
gold  leaf  communicating  with  the  ground. 

When  the  knob  is  touched  with  a  body  charged  with  either  kind 
of  electricity,  the  leaves  diverge ;  usually,  however,  the  apparatus 
is  charged  by  induction  thus  : 

If  an  electrified  body,  a  stick  of  sealing  wax  rubbed  with  flannel 
for  instance,  be  brought  near  the  knob,  it  will  decompose  the 
natural  electricity  of  the  system,  attracting  to  the  knob  the  fluid  of 

F  F2 


436  Frictional  Electricity.  [415- 

the  opposite  kind  and  retaining  it  there,  and  repelling  the  electricity 
of  the  same  kind  to  the  gold  leaves,  which  consequently  diverge. 
In  this  way,  the  presence  of  an  electrical  charge  is  ascertained,  but 
not  its  quality. 

To  ascertain  the  kind  of  electricity  the  following  method  is 
pursued  :  If,  while  the  instrument  is  under  the  influence  of  the 
body,  which  we  will  suppose  has  a  negative  charge,  the  knob  be 
touched  by  the  finger,  the  negative  electricity  decomposed  in  in- 
duction passes  off  into  the  ground,  and  the  previously  divergent 
leaves  will  collapse  :  there  only  remains  positive  electricity  retained 
in  the  knob  by  induction  from  the  sealing  wax.  If  now  the  finger  be 
first  removed,  and  then  the  electrified  body,  the  positive  electricity 
previously  retained  by  the  sealing  wax  will  spread  over  the  system, 
and  cause  the  leaves  to  diverge  with  positive  electricity.  If  now, 
while  the  system  is  charged  with  positive  electricity,  a  positively 
electrified  body,  as,  for  example,  an  excited  glass  rod,  be  ap- 
proached, the  leaves  will  diverge  more  widely  ;  for  the  electricity 
of  the  same- kind  will  be  repelled  to  the  extremities.  If,  on  the 
contrary,  an  excited  shellac  rod  be  presented,  the  leaves  will  tend 
to  collapse,  the  fluid,  with  which  they  are  charged,  being  attracted 
by  the  opposite  electricity.  Hence  we  may  ascertain  the  kind  of 
electricity,  either  by  imparting  to  the  electroscope  electricity  from 
the  body  under  examination,  and  then  bringing  near  it  a  rod 
charged  with  positive  or  negative  electricity  ;  or  the  electroscope 
may  be  charged  with  a  known  kind  of  electricity,  and  the  electrified 
body  in  question  brought  near  the  electroscope. 

It  has  been  proposed  to  use  the  electroscope  as  an  electrometer, 
or  measurer  of  electricity,  by  measuring  the  angle  of  divergence  of 
the  leaves.  This  is  done  by  placing  behind  them  a  graduated 
scale.  There  are,  however,  many  objections  to  such  a  use,  and  it 
is  rarely  employed  for  this  purpose. 


-416] 


Insulating  Stool. 


457 


CHAPTER   III. 

ELECTRICAL  EXPERIMENTS. 

416.  Electrical  spark. —  One  of  the  first  experiments  which  is 
made  by  those  who  see  an  electrical  machine  at  work  for  the  first 
time  is  that  of  taking  from  it  an  electrical  spark  by  bringing  the 
hand  near  the  conductor.  The  positive  electricity  of  the  conductor 
acting  inductively  on  the  neutral  fluid  of  the  body  decomposes  it, 


Fig.  344- 


Fig.  345- 


Fig.  346- 


repelling  the  positive  and  attracting  the  negative  fluid.  When  the 
tension  of  the  opposed  electricities  is  sufficiently  great  to  over- 
come the  resistance  of  the  air,  they  recombine  with  a  smart  crack 
and  a  spark.  The  spark  is  instantaneous,  and  is  accompanied  by 
a  sharp  prickly  sensation,  more  especially  with  a  powerful  machine. 
Us  shape  varies.  When  it  strikes  at  a  short  distance,  it  is  recti- 


438 


Frictional  Electricity. 


[416- 


linear,  as  seen  in  fig.  344.  Beyond  two  or  three  inches  in  length, 
the  spark  becomes  irregular,  and  has  the  form  of  a  sinuous  curve 
with  branches  (fig.  345).  If  the  discharge  is  very  powerful,  the 
spark  takes  a  zigzag  shape  (fig.  346).  These  two  latter  appearances 
are  seen  in  the  lightning  discharge. 

417.  Insulating  stool. — A  spark  may  be  taken  from  the  human 
body  by  the  aid  of  the  insulating  stool,  which  is  simply  a  low  stool 


Fig.  347- 

with  stout  varnished  glass  legs.  The  person  standing  on  this  stool 
touches  the  prime  conductor,  and  as  the  human  body  is  a  good  con- 
ductor, the  electrical  fluid  is  distributed  over  its  surface  as  over  an 
ordinary  insulated  metallic  conductor  (fig.  347).  The  hair  diverges 
in  consequence  of  repulsion,  a  peculiar  sensation  is  felt  on  the  face, 
and  if  another  person,  standing  on  the  ground,  presents  his  hand 
to  any  part  of  the  body,  a  smart  crack  with  a  pricking  sensation  is 
produced. 

418.  Electrical  chimes. — The  electrical  chimes  is  a  bell  work 


-420] 


Electrical  Chimes. 


439 


which  is  worked  by  electrical  attraction  and  repulsion.  It  consists 
of  three  metal  bells  suspended  to  a  horizontal  brass  rod,  m,  which  is 
connected  with  the  electrical  machine  (fig.  348).  The  two  bells,  b 
and  c,  are  suspended  by  light  metal  chains  ;  the  middle  one  is  sus- 
pended by  silk,  and  is  moreover  connected  with  the  ground  by  a 
chain.  Between  the  bells  are  two  small  hollow  copper  balls  sus- 
pended by  silk  threads  to  which  they  are  attached.  When  the 


Fig.  348. 

machine  is  worked  these  small  copper  balls  are  attracted  by  the 
electricity  which  passes  to  the  bells,  b  and  c,  and  strike  against 
them;  but  being  at  once  repelled  they  strike  against  the  middle  bell, 
to  which  they  give  up  their  electricity,  which  thus  passes  into  the 
ground.  They  are  then  again  attracted,  again  repelled,  and  so  on 
as  long  as  the  machine  is  at  work. 

419.  Dancing:  puppets. — This,  like  the  chimes,  is  an  application 
of  the  attractions  and  repulsions  of  electrified  bodies.     It  consists 
in  placing  a  small,  very  light  figure  of  pith,  loaded  at  the  feet, 
between  two  metal  discs,  one  connected  with  the  ground  and  the 
other  with  the  electrical  machine  (fig.  349).     As  soon  as  this  latter 
becomes  charged,  the  small  puppet  is  successively  attracted  and 
repelled  from  one  to  the  other  disc,  as  if  it   executed  of  its  own 
proper  action  a  series  of  jumps. 

420.  Electrical  whirl  or  vane. — The  electrical  'whirl  or  vane 
consists  of  four  to  six  wires,  terminating  in  points,  all  bent  in  the 


440 


Fractional  Electricity. 


[420- 


same  direction,  and  fixed  in  a  central  cap,  which  rotates  on  a 
pivot  (fig.  350).     When  the  apparatus  is  placed  on  the  conductor, 


Fig-  349- 

and  the  machine  worked,  the  whirl  begins  to  revolve  in  a  direction 
opposite  that  of  the  points.  This  motion  is  not  analogous  to  that 
of  the  hydraulic  tourniquet  (79).  It  is  not  caused  by  a  flow  of 
material  fluid,  but  is  due  to  a  repulsion  between  the  electricity  of 
the  points  and  that  which  they  impart  to  the  air  by  conduction. 
The  electricity,  being  accumulated  on  the  points  in  a  high  state  of 
tension,  passes  into  the  adjacent  air,  and  imparting  thus  a  charge 
of  electricity,  repels  this  electricity  while  it  is  itself  repelled.  That 
this  is  the  case,  is  evident  from  the  fact  that,  on  approaching 
the  hand  to  the  whirl  while  in  motion,  a  slight  draught  is  felt,  due 
to  the  movement  of  the  electrified  air  ;  while  in  vacuo  the  appa- 
ratus does  not  act  at  all.  This  draught  or  wind  is  known  as  the 
electrical  aura. 

When  the  electricity  thus  escapes  by  a  point,  the  electrified  air  is 


-421] 


Electrical  Egg. 


441 


repelled  so  strongly  as  not  only  to  be  perceptible  to  the  hand,  but 
also  to  engender  a  current  strong  enough  to  blow  out  a  candle. 
The  same  effect  is  produced  by 
placing  a  taper  on  the  con- 
ductor, and  bringing  near  it  a 
pointed  wire  held  in  the  hand. 
The  current  arises,  in  this  case, 
from  the  contrary  fluid,  which 
escapes  by  the  point  under  the 
influence  of  the  machine. 

The  electrical  orrery  and  the 
electrical  inclined  plane  are 
analogous  to  these  pieces  ot 
apparatus. 

421.  Electric  eggr. — The  in- 
fluence of  the  pressure  of  the 
air,  or  rather  of  its  nonconduc- 
tivity,  on  the  electric  light,  may 
be  studied  by  means  of  the 
electric  egg.  This  consists  of  an 


Fig-  351. 


442 


Frictional  Electricity. 


[421- 


ellipsoidal  glass  vessel  (fig  351),  with  metallic  caps  at  each  end. 
The  lower  cap  is  provided  with  a  stopcock,  so  that  it  can  be 
screwed  into  an  air-pump,  and  also  into  a  heavy  metal  foot.  The 
upper  metal  rod  moves  up  and  down  in  a  leather  stuffing  box  ; 
the  lower  one  is  fixed  to  the  cap.  An  almost  complete  vacuum 
having  been  made,  the  stopcock  is  turned,  and  the  vessel  screwed 
into  its  foot ;  the  upper  part  is  then  connected  with  a  powerful 
electrical  machine,  and  the  lower  one  with  the  ground.  On  work- 
ing the  machine,  the  globe  becomes  filled  with  a  feeble  violet  light 


Fig.  352. 

continuous  from  one  end  to  the  other,  and  resulting  from  the  recom- 
position  of  the  positive  fluid  of  the  upper  cap  with  the  negative  of 
the  lower.  If  the  air  be  gradually  allowed  to  enter  by  opening 


-422] 


Luminous  Globe  and  Tube. 


443 


the  stopcock,  the  resistance  increases,  and  the  light  which  ap- 
peared continuous,  white,  and  brilliant,  is  now  only  seen  as  an 
ordinary  spark. 

422.  Magic  pane. — The  magic  pane  consists  of  a  glass  plate, 
one  side  of  which  is  covered  with  several  folds  of  tinfoil,  arranged 
so  as  to  form  a  series  of  metallic  bands,  arranged  parallel  and  close 
to  each  other.  The  pane  is  supported  vertically  by  two  glass 
rods,  and  the  upper  end  of  the  tinfoil  is  connected  with  the  elec- 
trical machine  by  a  conductor,  and  the  lower  one  with  the  ground 


Fig.  353- 


by  a  chain.  In  this  condition,  if  the  machine  be  worked,  the  elec- 
tricity will  pass  into  the  ground  by  the  tinfoil,  without  any  inter- 
ruption ;  but  if  a  series  of  breaks  are  made  in  the  tinfoil  by  cutting 


444 


Frictional  Electricity. 


[422- 


it  away  with  a  penknife,  a  spark  appears  at  each  break  ;  and  if  these 
breaks  be  so  arranged  as  to  represent  a  given  object,  a  flower,  or  a 
monument,  or  words,  these  objects  are  reproduced  in  a  line  of  fire 
when  the  electrical  machine  is  set  to  work.  This  experiment  is 
really  due  to  the  prodigious  velocity  of  electricity,  which  is  not  less 
than  about  190,000  miles  in  a  second.  Hence,  in  the  above  experi- 
ment, although  the  sparks  are  really  successive,  they  follow  each 
other  with  such  rapidity  as  to  seem  continuous. 

423.  Luminous    globe  and  tube. — The   luminous  globe   is  a 
glass  globe  lined  on  the  inside  with  a  series  of  small  lozenges  of 
tinfoil  placed  very  near  each  other  without  actually  touching.     The 
first  plate  is  connected  with  an  electrical  machine  at  work,  and  the 
last  with  the  ground,  upon  which  a  series  of  bright  sparks  appears 
at  each  break  in  the  metallic  conductor  (fig.  353). 

If  the  small  metal  plates  are  arranged  inside  a  spiral  glass  lustre 
from  one  end  to  the  other,  this  arrangement  forms  a  luminous  tube. 

424.  Volta's  cannon. — This  is  not  merely  interesting  as  an  ex- 
periment, but  also  as  demonstrating  an  important  fact,  namely,  that 


Fig-  354- 

the  electrical  spark  can  establish  chemical  action.     Thus,  water  is 
formed  of  two  gases,  hydrogen  and  oxygen,  in  the  ratio  of  one  volume 


-425]  Condensation  of  Electricity.  445 

of  the  latter  to  two  volumes  of  the  former.  Now,  when  an  electrical 
spark  is  passed  through  a  mixture  of  these  two  gases,  they  combine 
in  their  proportions,  and  form  water.  This  combination  is,  moreover, 
attended  by  a  bright  flash  of  light  and  a  loud  report,  the  latter  being 
due  to  the  expansive  force  of  aqueous  vapour,  due  to  the  high  tem- 
perature produced  by  the  combination. 

On  this  property  which  mixtures  have  of  detonating  by  the  elec- 
trical spark,  Volta's  cannon,  represented  in  fig.  354,  is  constructed. 
It  is  a  small  brass  cannon  resting  on  an  insulating  support.  In  the 
touchhole  is  a  small  glass  tube,  and  in  this  a  brass  wire  with  a 
small  knob  at  each  end  ;  one  of  which  knobs  is  on  the  outside,  and 
the  other  very  near  the  inside  of  the  cannon  but  not  touching  it. 
Having  introduced  a  mixture  of  two  parts  of  hydrogen  and  one  of 
oxygen,  the  cannon  is  closed  by  a  cork,  and  is  connected  with  the 
ground  by  a  metal  chain.  If  then  the  charged  disc  of  the  electro- 
phorus  be  approached,  a  spark  passes  to  the  small  knob,  and  at  the 
same  time  inside  the  cannon.  This  latter  causes  the  two  gases  to 
combine  with  a  violent  explosion,  which  drives  out  the  cork. 


CHAPTER   IV. 

CONDENSATION   OF   ELECTRICITY. 

425.  Electrical  condensers. — Condensers  are  apparatus  by 
which  electricity  may  be  accumulated.  Their  shape  is  greatly 
varied,  but  they  are  all  composed  essentially  of  two  insulated  con- 
ductors separated  by  a  non-conductor,  and  their  working  is  an 
application  of  the  action  of  induction.  Epinus's  condenser  con- 
sists of  two  metal  plates,  A  and  B,  insulated  by  being  supported  on 
glass  legs  (fig.  355) ;  between  them  is  a  pane  of  ordinary  glass,  of 
somewhat  larger  diameter  than  that  of  the  plates,  A  and  B,  which 
are  about  six  inches  in  diameter.  The  legs  can  be  moved  along  a 
support,  and  fixed  in  any  position. 

In  explaining  the  action  of  the  condenser,  it  will  be  convenient 
to  call  that  side  of  the  metal  plate  nearest  the  glass  the  anterior,  and 
the  other  the  posterior,  side.  And  first  let  A  be  at  such  a  distance 
from  B  as  to  be  out  of  the  sphere  of  its  action.  The  plate  B,  which 
is  then  connected  with  the  conductor  of  the  electrical  machine,  takes 
its  maximum  charge,  which  is  distributed  equally  on  its  two  faces, 


446 


Frictional  Electricity. 


[425- 


and  the  pendulum  diverges  widely.     If  the  connection  with  the 
machine  be  interrupted,  nothing  would  be  changed  ;  but  if  the  plate, 


Fig.  355- 

A,  be  slowly  approached,  its  neutral  fluid  being  decomposed  by  the 


Fig.  356- 


influence  of  B,  the  negative  is  accumulated  on  its  anterior  face,  «, 
l(fig.  357),  and  the  positive  passes  into  the  ground.     But  as  the 


-425]         Slow  and  Instantaneous  Discharge. 


447 


negative  electricity  of  the  plate,  A,  reacts  in  its  turn  on  the  positive 
of  the  plate,  B,  the  latter  fluid  ceases  to  be  equally  distributed  on 
both  faces,  and  is  accumulated  on  its  anterior  face,  m.  The  pos- 
terior face,  p,  having  thus  lost  a  portion  of  its  electricity,  its  tension 
has  diminished,  and  is  no  longer  equal  to  that  of  the  machine,  and 
the  pendulum,  b,  diverges  less  widely.  Hence  B  can  receive  a  fresh 
quantity  from  the  machine,  which  acting  as  just  described,  decom- 
poses by  induction  a  second  quantity  of  neutral  fluid  on  the  plate, 
A.  There  is  then  a  new  accumulation  of  negative  fluid  on  the 
face,  n,  and  consequently  of  positive  fluid  on  m.  But  each  time 
the  machine  gives  off  electricity  to  the  plate  B,  only  a  proportion  of 
this  passes  to  the  face,  m,  the  other  remaining  on  the  face,  p  ;  the 
tension  here,  therefore,  continues  to  increase  until  it  equals  that  of 
the  machine.  From  this  moment  equilibrium  is  established,  and  a 
limit  to  the  charge  attained, 
which  cannot  be  exceeded.  The 
quantity  of  electricity  accumula- 
ted now  on  the  two  faces,  m  and 
72,  is  very  considerable,  and  yet  the 
pendulum  diverges  just  as  much 
as  it  did  when  A  was  absent  and 
no  more  ;  in  fact,  the  tension  at 
p,  is  just  what  it  was  then,  name- 
ly, that  of  the  machine. 

The  accumulation  of  electri- 
city in  condensers  was  formerly  explained  by  saying  that  the  elec- 
tricity of  the  condensing  plate,  A,  neutralised  the  contrary  electricity 
of  the  collecting  plate,  and  it  was  because  the  electricity  on  this 
latter  was  then  dissimulated  or  latent  that  it  could  receive  a  fresh 
supply.  But  from  what  has  been  said,  it  is  unnecessary  to  recur  to 
any  special  hypothesis  as  to  the  state  of  electricity  to  explain  the 
theory  of  condensers. 

When  the  condenser  is  charged,  that  is,  when  the  opposite  elec- 
tricities are  accumulated  on  the  anterior  faces,  connection  with 
the  ground  is  broken  by  raising  the  wires.  The  plate  A  is  charged' 
with  negative  electricity,  but  simply  on  its  anterior  face  (fig.  356), 
the  other  side  being  neutral.  The  plate  B,  on  the  contrary,  is  elec- 
trified on  both  sides,  but  unequally  ;  the  accumulation  is  only  on 
its  anterior  face,  while  on  the  posterior,  p,  the  tension  is  simply 
equal  to  that  of  the  machine  at  the  moment  the  connections  are 
interrupted.  In  fact  the  pendulum,  b,  diverges  and  a  remains 


Fig.  357- 


448  Frictional  Electricity.  [425- 

vertical.  But,  if  the  two  plates  are  removed,  the  two  pendulums 
diverge  (fig.  357),  which  is  owing  to  the  circumstance  that,  as  the 
plates  no  longer  act  on  each  other,  the  positive  fluid  is  equally  dis- 
tributed on  the  two  faces  of  the  plate  B,  and  the  negative  on  those 
of  the  plate  A. 

426.  Slow  discharge  and  instantaneous    discharge. — While 
the  plates,  A  and  B,  are  in  contact  with  the  glass  (fig.  356),  and  the 
connections  interrupted,  the  condenser  may  be  discharged,  that  is, 
restored  to  the  neutral  state,  in  two  ways  ;    either  by  a  slow  or  by 
an  instantaneous  discharge.     To  discharge  it  slowly,  the  plate  B, 
that  is,  the  one  containing  an  excess  of  electricity  is  touched  with 
the  finger  ;  a  spark  passes,  all  the  electricity  on  p  escapes  into  the 
ground,  the  pendulum,  £,  falls,  but  a  diverges.     For  B  having  lost 
part  of  its  electricity  only  retains  on  the  face,  m,  that  held  by  the 
inductive  influence  of  the  negative  on  A.     But  the  quantity  thus 
retained  at  B  is  less  than  that  on  A  :  this  has  free  electricity,  which 
makes  the  pendulum,  a,  diverge ;  and,  if  it  now  be  touched,  a  spark 
passes,  the  pendulum,  a,  sinks  while  b  rises,  and  so  on  by  continuing 
to  touch  alternately  the  two  plates.    The  discharge  only  takes  place 
slowly  ;  in  very  dry  air  it  may  require  several  hours.     If  the  plate, 
A,  were  touched  first,  no  electricity  would  be  removed,  for  all  it  has 
is  retained  by  that  of  the  plate  B.     To  remove  the  total  quantity  of 
electricity  by  the  method  of  alternate  contacts,  an  infinite  number 
of  such  contacts  would  theoretically  be  required. 

To  obtain  an  instantaneous  discharge  one  hand  may  be  placed 
on  one  plate,  and  the  second  touched  with  the  other  hand  ;  a  violent 
shock  is  then  felt,  far  more  violent  than  that  produced  by  the 
electrical  machine.  To  avoid  this  a  discharging  rod  is  used,  which 
consists  of  two  bent  stout  brass  wires  terminating  in  knobs  and 
joined  by  a  hinge.  If  this  be  held  in  the  hand  as  represented  in 
fig.  359,  and  one  knob  be  applied  to  one  plate  of  the  condenser 
while  the  arc  is  bent,  so  that  the  second  touches  the  other  plate. 
Just  as  this  is  on  the  point  of  touching  a  spark  passes,  which  is 
due  to  the  reunion  of  the  two  electricities  accumulated  on  the  con- 
denser ;  no  shock  is  felt,  for  the  recombination  does  not  take  place 
through  the  arms  and  body  of  the  experimenter,  but  through  the 
metallic  arc,  which  is  a  far  better  conductor. 

427.  limit  of  the  charge  of  condensers. — The  quantity  of  elec- 
tricity which  can  be  accumulated  on  each  plate  is,  other  things  being 
equal,  proportional  to  the  tension  of  the  electricity  on  the  conductor, 
and  to  the  surface  of  the  plates  :  it  decreases  as  the  insulating  plate 


-428] 


Lcyden  Jar. 


449 


is  thicker,  and  it  differs  with  the  specific  inductive  capacity  of  the 
substance.  Two  causes  limit  the  quantity  of  electricity  which  can 
be  accumulated.  First,  that  the  electric  tension  of  the  collecting 
plate  gradually  increases,  and  ultimately  equals  that  of  the  machine, 
which  cannot,  therefore,  impart  any  free  electricity.  The  second 
cause  is  the  imperfect  resistance  which  the  insulating  plate  offers  to 
the  recombination  of  the  two  opposite  electricities  ;  for  when  the 
force  which  impels  the  two  fluids  to  recombine  exceeds  the  resist- 
ance offered  by  the  insulating  plate,  it  is  perforated,  and  the  con- 
trary fluids  unite. 

428.  The  Xieyden  jar. — The  Ley  den  jar,  or  flask,  so  called  from 
the  town  of  Leyden,  where  it  was  invented,  is  essentially  a  conden- 
ser, only  differing  in  shape  from  that  which  has  been  described.  It 


Fig.  358. 

was  accidentally  discovered  in  1746  by  Muschenbrock,  a  Dutch 
physician.  Wishing  to  electrify  water  contained  in  a  flask,  he 
passed  through  the  cork  a  wire  which  he  presented  to  the  conductor 
of  the  machine.  After  holding  it  in  this  position  for  some  time  he 
was  on  the  point  of  removing  the  rod  with  the  other  hand  when  he 
received  in  the  arms  and  breast  a  shock  so  violent  that  it  was  two 
days  before  he  recovered  from  the  effects  ;  and  writing  to  his  friend 
Reaumur  he  said,  he  would  not  repeat  the  experiment  for  the  whole 
kingdom  of  France. 

The  fact  thus  discovered  caused  probably  a  greater  sensation 
throughout  Europe  than  any  other  one  has  ever  done.  It  was  re- 
peated innumerable  times,  and  the  apparatus,  after  successive  modi- 
fications and  improvements,  acquired  its  present  form.  It  is  not 

G  G 


450 


Frictional.  Electricity. 


[428- 


difficult  to  see  that  the  above  experiment  is  a  case  of  condensation. 
The  liquid  in  the  flask  acts  as  a  collector,  the  hand  acts  as  a  con- 
densing plate,  and  the  insulating  plate  is  formed  by  the  material  of 
the  flask  itself. 

The  ordinary  form  of  the  Leyden  jar  consists  of  a  glass  bottle  of 
any  convenient  size,  the  interior  of  which  is  either  coated  with  tin- 
foil or  filled  with  thin  leaves  of  copper,  or  with  gold  leaf.  Up  to  a 
certain  distance  from  the  neck  the  outside  is  coated  with  tinfoil. 
The  neck  is  provided  with  a  cork,  through  which  passes  a  brass 


Fig.  359- 

rod,  which  terminates  at  one  end  in  a  knob,  and  communicates  with 
the  metal  in  the  interior.  The  metallic  coatings  are  called  respec- 
tively the  internal  and  external  armatures  or  coatings.  Like  the 
condenser,  the  jar  is  charged  by  connecting  one  of  the  armatures 
with  the  ground,  and  the  other  with  the  source  of  electricity.  When 
it  is  held  in  the  hand  by  the  outer  coating,  and  the  knob  pre- 
sented to  the  conductor  of  the  machine,  positive  electricity  is  accu- 
mulated on  the  inner,  and  negative  electricity  on  the  outer  coating. 
The  reverse  is  the  case  if  the  jar  is  held  by  the  knob,  and  the 
outer  coating  presented  to  the  machine.  The  action  of  the  jar 
is  the  same  as  that  of  the  condenser,  and  all  that  has  been  said 
of  this  applies  to  the  jar,  substituting  the  two  armatures  for  the  two 
plates  A  and  B,  of  the  condenser. 


-429] 


Ley  den  Jar. 


451 


To  charge  the  jar  it  is  held  in  the  hand  as  represented  in  fig.  358, 
and  the  knob  is  applied  to  an  electrical  machine,  which  is  at  work. 
The  positive  electricity  of  the  machine  acting  inductively  through 
the  sides  of  the  glass  on  the  tinfoil  and  on  the  hand,  condenses  a 
large  quantity  of  electricity. 

Like  the  condenser,  the  Leyden  jar  may  be  discharged  either 
slowly  or  instantaneously.  For  the  latter  it  is  held  in  the  hand  by 
the  outside  coating,  and  the  two  coatings  are  then  connected  by 
means  of  the  simple  discharger  (fig.  359).  Care  must  be  taken  to 
touch  first  the  external  coating  with  the  discharger,  otherwise  a 
smart  shock  will  be  felt.  To  discharge  it  slowly  the  jar  is  placed 
on  an  insulated  plate,  and  first  the  internal  and  then  the  external 
coating  touched,  either  with  the  hand  or  with  a  metallic  conductor. 
A  slight  spark  is  seen  at  each  discharge. 

Fig.  360  represents  a  very  pretty  experiment  for  illustrating  the 
slow  discharge.  The  rod  terminates  in  a  small  bell,  d,  and  the 
outside  coating  is  connected  with  an  upright  metallic  support,  on 
which  is  a  similar  bell,  e.  Between  the  two  bells  a  light  copper  ball 
is  suspended  by  a  silk  thread.  The 
jar  is  then  charged  in  the  usual 
manner,  and  placed  on  the  support, 
m.  The  internal  armature  contains 
a  quantity  of  free  electricity  ;  the 
pendulum  is  attracted  and  imme- 
diately repelled,  striking  against 
the  second  bell,  to  which  it  imparts 
its  free  electricity.  Being  now 
neutralised  it  is  again  attracted  by 
the  first  bell,  and  so  on  for  some 
time,  especially  if  the  air  be  dry, 
and  the  jar  pretty  large. 

429.  Electric  batteries. — The 
charge  which  a  Leyden  jar  can 
take  depends  on  the  extent  of  the 
coated  surface,  and  for  small  thick-  Fig  36o 

nesses  is  inversely  proportional  to 

the  thickness  of  the  insulator.  Hence  the  larger  and  thinner 
the  jar  the  more  powerful  the  charge.  But  very  large  jars  are  ex- 
pensive, and  liable  to  break  ;  and,  when  too  thin,  the  accumulated 
electricities  are  apt  to  discharge  themselves  through  the  glass  es- 
pecially if  it  is  not  quite  homogeneous.  Leyden  jars  have  usually 


452 


Frictional  Electricity. 


[429- 


from  \   to  3  square  feet  of  coated  surface.     For  more  powerful 
charges  electric  batteries  are  used. 

An  electric  battery  consists  of  a  series  of  Leyden  jars,  whose  in- 
ternal and  external  coatings  are  respectively  connected  with  each 
other  (fig.  361).  They  are  usually  placed  in  a  wooden  box  lined  on 
the  bottom  with  tinfoil.  This  lining  is  connected  with  two  metal 
handles  in  the  sides  of  the  box.  The  internal  coatings  are  con- 
nected with  each  other  by  metallic  rods,  and  the  battery  is  charged 
by  placing  the  internal  coatings  in  connection  with  the  prime  con- 
ductor, while  the  external  coatings  are  connected  with  the  ground 
by  means  of  a  chain  fixed  to  the  handles.  A  quadrant  electrometer 


Fig.  361. 

fixed  to  the  jar  serves  to  'indicate  the  charge  of  the  battery.  Al- 
though there  is  a  large  quantity  of  electricity  accumulated  in  the 
apparatus  the  divergence  is  not  great,  for  it  is  simply  due  to  the  free 
electricity  on  the  internal  coating.  The  number  of  jars  is  usually 
four,  six,  or  nine.  The  larger  and  more  numerous  they  are,  the 
longer  is  the  time  required  to  charge  the  battery,  but  the  effects  are 
so  much  the  more  powerful. 

When  a  battery  is  to  be  discharged,  the  coatings  are  connected 
by  means  of  the  discharging  rod,  the  outside  coating  being  touched 
first.  Great  care  is  required,  for  with  large  batteries  serious  acci- 
dents may  be  produced,  resulting  even  in  death. 

430.  Condensing:  electroscope. — We  shall  conclude  the  study 


-430] 


Condensing  Electroscope. 


453 


of  condensers  by  an  application  which  Volta  made  of  this  principle 
to  the  ordinary  gold  leaf  electroscope,  by  which  a  far  greater  degree 
of  delicacy  is  attained  (fig.  362).  The  rod  to  which  the  gold  leaves 
are  affixed,  terminates  in  a  disc  instead  of  in  a  knob,  and  there  is 
another  disc  of  the  same  size  provided  with  an  insulating  glass 
handle.  The  discs  are  covered  with  a  layer  of  insulating  shellac 
varnish  (fig.  362). 


Fig.  362. 


Fig.  363. 


To  render  very  small  quantities  of  electricity  perceptible  by  this 
apparatus,  one  of  the  plates,  which  thus  becomes  the  collecting 
plate,  is  touched  with  the  body  under  examination.  The  other 
plate,  the  condensing  plate,  is  connected  with  the  ground,  by 
touching  it  with  the  finger.  The  electricity  of  the  body,  being 
diffused  over  the  collecting  plate,  acts  inductively  through  the 
varnish  on  the  neutral  fluid  of  the  other  plate,  attracting  the 
opposite  electricity,  but  repelling  that  of  like  kind.  The  two 
electricities  thus  become  accumulated  on  the  two  plates  just  as  in 
Epinus's  condenser,  but  there  is  no  divergence  of  the  leaves,  for  the 
opposite  electricities  counteract  each  other.  The  finger  is  now 


454  Fractional  Electricity.  [430- 

removed,  and  then  the  source  of  electricity,  and  still  there  is  no 
divergence  ;  but  if  the  upper  plate  be  raised  (fig.  363),  the  neutrali- 
sation ceases,  and  the  electricity  being  free  to  move  diffuses  itself 
over  the  rod  and  the  leaves,  which  then  diverge  widely.  The 
delicacy  of  the  apparatus  is  increased  by  adapting  to  the  foot  of 
the  apparatus  two  metallic  rods,  terminating  in  knobs,  for  these 
knobs  being  excited  by  induction  from  the  gold  leaves  react  upon 
them. 


CHAPTER  V. 

VARIOUS   EFFECTS   OF  ACCUMULATED   ELECTRICITY. 

431.  Effects  of  the  electric  discharge. — The  recombination  of 
the  two  electricities  which  constitutes  the  electrical  discharge  may 
be  either  continuous  or  sudden  ;  continuous,  or  of  the  nature  of  a 
current,  as  when  the  two  conductors  of  a  cylinder  machine  are 
joined  by  a  chain  or  a  wire  ;  and  sudden,  as  when  the  opposite 
electricities  accumulate  on  the  surface  of  two  adjacent  conductors, 
till   their  mutual   attraction   is    strong   enough  to   overcome   the 
intervening  resistances,  whatever  they  may  be.     But  the  difference 
between  a  sudden  and  a  continuous  discharge   is  one  of  degree 
and  not  of  kind,  for  there  is  no   such  thing  as  an  absolute  non- 
conductor, and  the  very    best   conductors,    the   metals,    offer    an 
appreciable  resistance   to   the   passage   of  electricity.      Still,   the 
difference  at  the  two  extremes  of  the  scale  is  sufficiently  great  to 
give  rise  to  a  wide  range  of  phenomena. 

The  phenomena  of  the  discharge  are  usually  divided  into  the 
physiological,  luminous,  mechanical,  magnetical,  and  chemical 
effects. 

432.  Physiological  effects.— The  physiological  effects  are  those 
produced  on  living  beings,  or  on  those  recently  deprived  of  life.     In 
the  first  case  they  consist  of  a  violent  excitement  which  the  electric 
fluid  exerts   on   the  sensibility   and  contractibility  of  the  organic 
tissues   through    which  it   passes ;  and   in   the   latter,   of  violent 
muscular  convulsions  which  resemble  a  return  to  life. 

The  shock  from  the  electrical  machine  has  been  already  no- 
ticed (416).  The  shock  taken  from  a  charged  Leyden,  jar  by 
grasping  the  external  coating  with  one  hand  and  touching  the  inner 
with  the  other,  is  much  more  violent,  and  has  a  peculiar  character. 


-432] 


Physiological  Effects  of  Electricity. 


455 


With  a  small  jar  the  shock  is  felt  in  the  elbow  ;  with  a  jar  of  about 
a  quart  capacity  it  is  felt  across  the  chest,  and  with  jars  of  still 
larger  dimensions  in  the  stomach. 

A  shock  may  be  given  to  a  larger  number  of  persons  simul- 
taneously by  means  of  the  Leyden  jar.  For  this  purpose  they 
must  form  a  chain  by  joining  hands.  If  then  the  first  touches  the 
outside  coating  of  a  charged  jar,  while  the  last  at  the  same  time 
touches  the  knob,  all  receive  a  simultaneous  shock,  the  intensity  of 


Fig.  364. 

which  depends  on  the  charge,  and  on  the  number  of  persons 
receiving  it.  Those  in  the  centre  of  the  chain  are  found  to 
receive  a  less  violent  shock  than  those  near  the  extremities.  The 
Abbe  Nollet  discharged  a  Leyden  jar  through  an  entire  regiment 
of  1,500  men,  all  of  whom  received  a  violent  shock  in  the  arms  and 
shoulders. 

With  large  Leyden  jars  and  batteries  the  shock  is  sometimes 
very   dangerous.      Priestley  killed  rats   with  batteries  of  7   feet 


456 


Frictional  Electricity. 


[432- 


coated  surface,  and  cats  with  a  battery  of  about  4^  square  yards 
coating. 

433.  luminous  effects.  Luminous  jar. — The  luminous  effects 
of  electricity  are  in  all  cases  due  to  the  combination  of  the  two 
fluids,  positive  and  negative.  Some  of  these  effects  have  already 
been  made  known  in  describing  the  electrical  egg  and  the  magic 
pane.  We  here  give  a  description  of  another  one. 

The  luminous  jar  (fig.  365)  is  a  Leyden  jar,  whose  outer  coating 
consists  of  a  layer  of  varnish  strewed  over  with  metallic  powder. 


Fig.  365. 


Fig.  366. 


A  strip  of  tin  fitted  on  the  bottom  is  connected  with  the  ground  by 
means  of  a  chain  ;  a  second  band  at  the  upper  part  of  the  coating 
has  a  projecting  part,  and  the  rod  of  the  bottle  is  curved  so  that 
the  knob  is  about  f  of  an  inch  distant  from  the  projection.  This 
bottle  is  suspended  from  the  machine,  and  as  rapidly  as  this  is 
worked  large  and  brilliant  sparks  pass  between  the  knob  and  the 
outer  coating,  illuminating  the  outside  of  the  apparatus. 


-434] 


Heating  Effects  of  Electricity. 


457 


434.  Heating:  effects. — Besides  being  luminous,  the  electric 
spark  is  a  source  of  intense  heat.  When  it  passes  through  inflam- 
mable liquids,  as  ether  or  alcohol,  it  inflames  them.  An  arrangement 
for  effecting  this  is  represented  in  fig.  366.  It  is  a  small  glass  cup 
through  the  bottom  of  which  passes  a  metal  rod,  terminating  in  a 
knob  and  fixed  to  a  metal  foot.  A  quantity  of  liquid  sufficient  to 
cover  the  knob  is  placed  in  the  vessel.  The  outer  coating  of  the 
jar  having  been  connected  with  the  foot  by  means  of  a  chain,  the 
spark  which  passes  when  the  two  knobs  are  brought  near  each 


other,  inflames  the  liquid.     With  ether  the  experiment  succeeds 
very  well,  but  alcohol  requires  to  .be  first  warmed. 

Coal  gas  may  also  be  ignited  by  means  of  the  electric  spark.  A 
person  standing  on  an  insulated  stool  places  one  hand  on  the 
conductor  of  a  machine  which  is  then  worked,  while  he  presents 
the  other  to  the  jet  of  gas  issuing  from  a  metallic  burner.  The 
spark  which  passes  ignites  the  gas.  This  experiment  may  be 


458  Frictional  Electricity.  [434- 

curiously  varied  by  igniting  the  gas  by  means  of  a  piece  of  ice  held 
in  the  hand. 

When  a  battery  is  discharged  through  a  metal  wire  it  becomes 
incandescent,  and  may  be  melted  or  even  volatilised  provided  the 
charge  be  sufficiently  powerful. 

For  this  experiment  an  apparatus  is  used  which  is  called  the 
universal  discharger,  for  it  may  be  employed  in  a  host  of  experi- 
ments on  the  electrical  discharge.  It  consists  (fig.  367)  of  two 
brass  rods,  A  and  B,  each  insulated  on  a  glass  stem.  These  rods 
can  slide  along  hinged  joints,  so  that  they  can  be  placed  at  any 
distance  from  each  other  and  inclined  in  any  direction.  Between 
them  is  a  small  table  support,  which  can  be  placed  at  any  height, 
and  which  is  intended  to  support  objects  which  are  to  be  sub- 
mitted to  the  action  of  the  discharge. 

To  melt  a  metal  wire  it  is  fixed  at  i  to  two  knobs  fastened  on  the 
rods,  then  connecting  one  of  these  by  means  of  a  chain  with  the 
outside  of  a  powerful  battery,  the  other  is  brought  in  contact  with 
the  inner  coating,  either  by  means  of  the  discharging  rod,  or  by  a 
chain  attached  to  a  metal  rod  fixed  on  a  glass  handle.  The 
moment  the  spark  passes  between  the  knob  and  the  battery,  the 
wire,  if  it  is  fine  enough,  is  melted  in  incandescent  globules,  and  is 
even  volatilised,  that  is,  converted  into  vapours  which  disappear  in 
the  atmosphere.  If  the  wire  is  thicker  it  simply  becomes  red  hot 
but  does  not  melt,  and  if  still  larger  it  is  merely  heated  without 
becoming  luminous. 

When  an  electric  discharge  is  sent  through  gunpowder  placed  on 
the  table  of  a  Henley's  discharger,  it  is  not  ignited,  but  is  projected 
in  all  directions.  But  if  a  wet  string  be  interposed  in  the  circuit,  a 
spark  passes  which  ignites  the  powder.  This  arises  from  the  re- 
tardation which  electricity  experiences  in  traversing  a  semi-con- 
ductor, such  as  a  wet  string  ;  for  the  heating  effect  is  proportional 
to  the  duration  of  the  discharge. 

435.  Electrical  portraits. — The  fusion  of  metals  by  the  electri- 
cal discharge  is  applied  to  make  what  are  called  electrical  portraits. 
For  this  purpose  a  thin  card  is  taken  of  the  shape  abm,  and  the 
design  to  be  copied  is  cut  out  :  a  sheet  of  tinfoil  is  fastened  on  the 
rest  of  the  card  at  a  and  b,  but  not  at  c.  A  leaf  of  gold  is  then 
placed  upon  the  design,  care  being  taken  that  it  touches  both  the 
pieces  of  tinfoil,  a  and  b.  The  lateral  portion  of  the  card,  ?;z,  is  then 
bent  over,  the  card  placed  on  a  silk  ribbon,  and  the  whole  pressed 
in  a  frame,  P.  When  the  discharge  is  passed  from  a  to  $,  the 


-437]  Mechanical  Effects.  459 

tinfoil  being  thicker  is  not  melted ;  but  the  gold  which  is  very  thin 
is  volatilised,  and  forms  on  the  ribbon  through  the  pattern  a  brown 
coating,  which  reproduces,  all  the  details  as  seen  in  R. 


Fig.  368. 

436.  Mechanical  effects. — The  mechanical  effects  are  the  violent 
lacerations,  fractures,  and  sudden  expansions  which  ensue  when  a 
powerful  discharge  is  passed  through  a  badly  conducting  substance. 
Glass  is  perforated,  wood  and  stones  are  fractured,  and  gases  and 
liquids  are  violently  disturbed.     The  mechanical  effects  of  elec- 
tricity may  be  demonstrated  by  a  variety  of  experiments.     The 
body  to  be  submitted  to  experiment  is  placed  on  the  plate,  N,  in   , 
contact  with  the  two  knobs  which  terminate  the  rods,  A  and  B,  so 
that  they  cannot  receive  the  discharge  without  transmitting  it  to 
the  object  on  the  table.     Thus,  for  instance,  if  a  piece  of  wood  is 
placed  so  as  to  be  struck  in  the  direction  of  the  fibres,  it  is  smashed 
into  pieces  the  moment  the  discharge  passes. 

Fig.  369  represents  an  arrangement  for  perforating  a  piece  of 
glass  or  card.  It  consists  of  two  glass  columns,  with  a  horizontal 
cross  piece,  in  which  is  a  pointed  conductor.  The  piece  of  glass, 
is  placed  on  an  insulating  glass  support,  in  which  is  placed  a  second 
conductor,  terminating  also  in  a  point,  which  is  connected  with  the 
outside  of  the  battery,  while  the  knob  of  the  inner  coating  is  brought 
near  the  other  knob.  When  the  discharge  passes  between  the  two 
conductors  the  glass  is  perforated.  The  experiment  only  succeeds 
with  a  single  jar  when  the  glass  is  very  thin  ;  otherwise  a  battery 
must  be  used. 

437.  Chemical  effects. — The  chemical  effects  are  the  decom- 
positions and  recombinations  effected  by  the  passage  of  the  elec- 
trical discharge.     When   two  gases  which  act  on  each  other  are 
mixed  in  the  proportion  in  which  they  combine,  a  single  spark  is 


460 


Frictional  Electricity. 


[437- 


generally  necessary  to  determine  their  combination  ;  but,  when 
either  of  them  is  in  great  excess,  a  succession  of  sparks  is  necessary. 
Priestley  found,  that  when  a  series  of  electric  sparks  was  passed 


Fig.  369. 

through  moist  air,  its  volume  diminished,  and  blue  litmus  intro- 
duced into  the  vessel  was  reddened.  This,  Cavendish  found,  was 
due  to  the  formation  of  nitric  acid. 

Among  the  chemical  effects  must  be  enumerated  the  formation 
of  ozone,  which  is  recognised  by  its  peculiar  odour  and  by  certain 
chemical  properties.  The  odour  is  perceived  when  electricity  issues 
through  a  series  of  points  from  a  conductor  into  the  air.  Its  true 
nature  is  not  accurately  known  :  some  regard  it,  and  with  great 
probability,  as  an  allotropic  modification  of  oxygen,  and  others  as  a 
teroxide  of  hydrogen. 

438.  Magnetic  effects. — By  the  discharge  of  a  large  Leyden  jar 
or  battery,  a  steel  wire  may  be  magnetised  if  it  is  laid  at  right 
angles  to  the  conducting  wire,  through  which  the  discharge  is  passed, 
either  in  contact  with  the  wire  or  at  some  slight  distance.  And 
even  with  less  powerful  discharges  a  steel  bar  or  needle  may  be  mag- 
netised by  placing  it  inside  a  tube  on  which  is  coiled  a  fine  insulated 
copper  wire.  On  passing  the  discharge  through  this  wire  the  steel 
becomes  magnetised. 


-440]  Atmospheric  Electricity.  461 


CHAPTER  VI. 

ATMOSPHERIC   ELECTRICITY.      THUNDER   AND   LIGHTNING. 

439.  Thunder  and  lightning  the  effect  of  electricity. — The 

first  physicists  who  observed  the  zigzag  motion  of  the  electric  spark, 
compared  it  to  the  gleam  of  lightning,  and  its  crackling  to  the 
sound  of  thunder.  But  Franklin,  by  the  aid  of  powerful  electrical 
batteries,  first  established  a  complete  parallelism  between  lightning 
and  electricity  ;  and,  in  a  memoir  published  in  1749,  he  indicated 
the  experiments  necessary  to  attract  electricity  from  the  clouds  by 
means  of  pointed  rods.  The  electric  fluid,  said  he,  in  concluding 
his  memoir,  is  attracted  by  points  ;  we  know  not  whether  lightning 
is  endowed  with  the  same  property  ;  but,  since  electricity  and  light- 
ning agree  in  all  other  respects,  it  is  probable  they  will  not  differ  in 
this  ;  and  the  experiment  should  be  made.  The  experiment  was 
tried  by  Dalibard  in  France  ;  and  Franklin,  pending  the  erection  of 
a  pointed  rod  on  a  spire  in  Philadelphia,  had  the  happy  idea  of  flying 
a  kite,  provided  with  a  metal  point,  which  could  reach  the  higher 
regions  of  the  atmosphere.  In  June,  1752,  during  stormy  weather, 
he  flew  the  kite  in  a  field  near  Philadelphia.  The  kite  was  flown 
with  ordinary  pack-thread,  at  the  end  of  which  Franklin  attached  a 
key,  and  to  the  key  a  silk  cord,  in  order  to  insulate  the  apparatus  ; 
he  then  fixed  the  silk  cord  to  a  tree,  and  having  presented  his  hand 
to  the  key,  at  first  he  obtained  no  spark.  He  was  beginning  to 
despair  of  success,  when,  rain  having  fallen,  the  cord  became  a 
good  conductor,  and  a  spark  passed.  Franklin,  in  his  letters, 
describes  his  emotion  on  witnessing  the  success  of  the  experiment 
as  being  so  great  that  he  could  not  refrain  from  tears. 

Franklin,  who  had  discovered  the  power  of  points  (422),  but  who 
did  not  understand  its  explanation,  imagined  that  the  kite  withdrew 
from  the  cloud  its  electricity  ;  it  is,  in  fact,  a  simple  case  of  induc- 
tion, and  depends  on  the  inductive  action  which  the  thunder-cloud 
exerts  upon  the  kite  and  the  cord. 

440.  Atmospheric  electricity. — In  order  to  ascertain  the 
presence  of  electricity  in  the  atmosphere,  many  forms  of  apparatus 
have  been  used.  To  observe  the  electricity  in  fine  weather,  when 
the  tension  is  generally  small,  an  electrometer  may  be  used,  as  de- 
vised by  Saussure  for  this  kind  of  investigation.  It  is  an  electro- 
scope similar  to  that  already  described,  but  the  rod  to  which  the 


462  Frictional  Electricity.  [440- 

gold  leaves  are  fixed,  is  surmounted  by  a  conductor  two  feet  in  length, 
and  terminating  either  in  a  knob  or  a  point.  To  protect  the  appa- 
ratus against  rain,  it  is  covered  with  a  metallic  shield,  four  inches  in 
diameter.  The  glass  case  is  square,  instead  of  being  round,  and  a 
divided  scale  on  its  inside  face  indicates  the  divergence  of  the  gold 
leaves. 

To  ascertain  the  electricity  of  the  atmosphere,  Saussure  also  used 
a  copper  ball,  which  he  projected  vertically  with  his  hand.  This 
ball  was  fixed  to  one  end  of  a  metallic  wire,  the  other  end  of  which 
was  attached  to  a  ring,  which  could  glide  along  the  conductor  ot 
the  electrometer.  From  the  divergence  of  the  gold  leaves  the 
electrical  condition  of  the  air  at  the  height  which  the  ball  had  at- 
tained could  be  determined.  M.  Becquerel,  in  experiments  made 
on  Mont  St.  Bernard,  improved  Saussure's  apparatus  by  substitut- 
ing for  the  knob  an  arrow,  which  was  projected  into  the  atmosphere 
by  means  of  a  bow.  A  gilt  silk  thread,  eighty-eight  yards  long,  was 
fixed  with  one  end  to  the  arrow,  while  the  other  was  attached  to  the 
stem  of  an  electroscope. 

Sometimes  also  kites  are  used,  provided  with  a  point,  and  con- 
nected by  means  of  a  gilt  cord  with  an  electrometer.  Captive 
balloons  are  also  similarly  used. 

A  good  collector  of  atmospheric  electricity  consists  of  a  fishing 
rod  with  an  insulating  handle,  which  projects  from  an  upper 
window.  At  the  top  is  a  bit  of  lighted  tinder  held  in  metal  forceps, 
the  smoke  of  which,  being  an  excellent  conductor,  conveys  the 
electricity  of  the  air  down  a  wire  attached  to  the  rod.  A  sponge 
moistened  with  alcohol,  and  set  on  fire,  is  also  an  excellent  con- 
ductor. 

441.  Ordinary  electricity  of  the  atmosphere. — By  means  of 
the  'different  apparatus  which  have  been  described,  it  has  been 
found  that  the  presence  of  electricity  in  the  atmosphere  is  not  con- 
fined to  stormy  weather,  but  that  the  atmosphere  always  contains 
free  electricity,  sometimes  positive  and  sometimes  negative.  When 
the  sky  is  cloudless,  the  electricity  is  always  positive,  but  it  varies  in 
intensity  with  the  height  of  the  locality,  and  with  the  time  of  day. 
The  intensity  is  greatest  in  the  highest  and  most  isolated  places. 
No  trace  of  positive  electricity  is  found  in  houses,  streets,  or  under 
trees  ;  in  towns,  positive  electricity  is  most  perceptible  in  large  open 
spaces,  on  quays,  or  on  bridges.  In  all  cases,  positive  electricity  is 
only  found  at  a  certain  height  above  the  ground.  On  flat  land  it 
only  becomes  perceptible  at  a  height  of  five  feet  ;  above  that  point  it 


-442]  Lightning.  463 

increases  according  to  a  law  which  is  not  made  out,  but  which  seems 
to  depend  on  the  hygrometric  state  of  the  air. 

When  the  sky  is  clouded,  the  electricity  is  sometimes  positive 
and  sometimes  negative.  It  often  happens  that  the  electricity 
changes  its  sign  several  times  in  the  course  of  the  day,  owing  to  the 
passage  of  an  electrified  cloud.  During  storms,  and  when  it  rains 
or  snows,  the  atmosphere  may  be  positively  electrified  one  day,  and 
negatively  the  next,  and  the  numbers  of  the  two  sets  of  days  are 
virtually  equal. 

The  electricity  of  the  ground  has  been  found  by  Peltier  to  be 
always  negative,  but  to  different  extents,  according  to  the  hygrome- 
tric state  and  temperature  of  the  air. 

Many  hypotheses  have  been  propounded  to  explain  the  origin  of 
the  atmospheric  electricity.  Some  have  ascribed  it  to  the  friction 
of  the  air  against  the  ground,  some  to  the  vegetation  of  plants,  or  to 
the  evaporation  of  water.  Some,  again,  have  compared  the  earth 
to  a  vast  voltaic  pile,  and  others  to  a  thermo- electrical  apparatus. 
Many  of  these  causes  may,  in  fact,  concur  in  producing  the  phe- 
nomena. 

442.  lightning. — This,  as  is  well  known,  is  the  dazzling  light 
emitted  by  the  electric  spark  when  it  shoots  from  clouds  charged 
with  electricity.  In  the  lower  regions  of  the  atmosphere  the  light 
is  white,  but  in  the  higher  regions,  where  the  air  is  more  rarefied,  it 
takes  a  violet  tint ;  as  does  the  spark  of  the  electrical  machine  in  a 
rarefied  medium  (421). 

The  flashes  of  lightning  are  sometimes  several  miles  in  length  ; 
they  generally  pass  through  the  atmosphere  in  a  zigzag  direction  ;  a 
phenomenon  ascribed  to  the  resistance  offered  by  the  air  condensed 
by  the  passage  of  a  strong  discharge.  The  spark  then  diverges 
from  a  white  line,  and  takes  the  direction  of  least  resistance.  In 
vacuo  electricity  passes  in  a  straight  line. 

Several  kinds  of  lightning-flashes  maybe  distinguished  :  i.  The 
zigzag  flashes,  which  move  with  extreme  velocity  in  the  form  of  a 
line  of  fire  with  sharp  outlines,  and  which  closely  resemble  the 
spark  of  an  electrical  machine.  2.  The  flashes  which,  instead  of 
being  linear,  like  the  preceding,  fill  the  entire  horizon  without  having 
any  distinct  shape.  This  kind,  which  is  most  frequent,  appears  to 
be  produced  in  the  cloud  itself,  and  to  illuminate  the  mass.  Another 
kind  is  called  heat  lightning,  because  it  illuminates  the  summer 
nights  without  the  presence  of  any  clouds  above  the  horizon,  and 
without  producing  any  sound.  The  most  probable  of  the  many  hy- 


464  Frictional  Electricity.  [442- 

potheses  which  have  been  proposed  to  account  for  its  origin,  is  that 
which  supposes  it  to  consist  of  ordinary  lightning  flashes,  which 
strike  across  the  clouds  at  such  distances  that  the  rolling  of  thunder 
cannot  reach  the  ear  of  the  observer.  There  are,  further,  the  light- 
ning flashes  which  appear  in  the  form  of  globes  of  fire.  These, 
which  are  sometimes  visible  for  as  much  as  ten  seconds,  descend 
from  the  clouds  to  the  earth  with  such  slowness  that  the  eye  can 
follow  them.  They  often  rebound  on  reaching  the  ground  ;  at  other 
times  they  burst  and  explode  with  a  noise  like  that  of  the  report  of 
many  cannon.  This  is  sometimes  known  as  globe  lightning, 

The  duration  of  the  light  of  the  first  three  kinds  does  not  amount 
to  a  thousandth  of  a  second,  as  has  been  determined  by  Mr.  Wheat- 
stone  by  means  of  a  rotating  wheel,  which  was  turned  so  rapidly 
that  the  spokes  were  invisible  ;  on  illuminating  it  by  the  lightning- 
flash,  its  duration  was  so  short  that  whatever  the  velocity  of  rotation 
of  the  wheel,  it  appeared  quite  stationary  ;  that  is,  its  displacement 
is  not  perceptible  during  the  time  the  lightning  exists. 

443.  Thunder. — Thunder  is  the  violent  report,  which  succeeds 
lightning  in  stormy  weather.  The  lightning  and  the  thunder  are 
always  simultaneous,  but  an  interval  of  several  seconds  is  always 
observed  between  these  two  phenomena,  which  arises  from  the  fact 
that  sound  only  travels  at  the  rate  of  about  1,100  feet  in  a  second 
(161),  while  the  passage  of  light  is  almost  instantaneous.  Hence  an 
observer  will  only  hear  the  noise  of  thunder  five  or  six  seconds,  for 
instance,  after  the  lightning,  according  as  the  distance  of  the 
thunder-cloud  is  five  or  six  times  1,100  feet.  The  noise  of  thunder 
arises  from  the  disturbance  which  the  electric  discharge  produces 
in  the  air.  Near  the  place  where  the  lightning  strikes,  the  sound  is 
dry  and  of  short  duration.  At  a  greater  distance  a  series  of  reports 
are  heard  in  rapid  succession.  At  a  still  greater  distance  the  noise, 
feeble  at  the  commencement,  changes  into  a  prolonged  rolling 
sound  of  varying  intensity.  Some  attribute  the  noise  of  the  rolling 
of  thunder  to  the  reflection  of  sound  from  the  ground  and  from  the 
clouds.  Others  have  considered  the  lightning  not  as  a  single  dis- 
charge, but  as  a  series  of  discharges,  each  of  which  gives  rise  to  a 
particular  sound.  But  as  these  partial  discharges  proceed  from 
points  at  different  distances,  and  from  zones  of  unequal  density,  it 
follows  not  only  that  they  reach  the  ear  of  the  observer  succes- 
sively, but  that  they  bring  sounds  of  unequal  density,  which  occa- 
sion the  duration  and  inequality  of  the  rolling.  The  phenomenon 
has  finally  been  ascribed  to  the  zigzags  of  lightning  themselves, 


-444]  Effects  of  Lightning.  465 

assuming  that  the  air  at  each  salient  angle  is  at  its  greatest 
compression,  which  would  produce  the  unequal  intensity  of  the 
sound. 

444.  Effects  of  lightning:. — The  lightning  discharge  is  the  elec- 
tric discharge  which  strikes  between  a  thunder-cloud  and  the 
ground.  The  latter,  by  the  induction  from  the  electricity  of  the 
cloud,  becomes  charged  with  contrary  electricity,  and  when  the 
tendency  of  the  two  electricities  to  combine  exceeds  the  resistance 
of  the  air,  the  spark  passes,  which  is  often  expressed  by  saying  that 
a  thunder-bolt  has  fallen.  Lightning  in  general  strikes  from  above, 
but  ascending  lightning  is  also  sometimes  observed  ;  probably  this 
is  the  case  when  the  clouds  being  negatively,  the  earth  is  positively 
electrified ;  for  experiments  seem  to  show  that  at  the  ordinary 
pressure  the  positive  electricity  passes  through  the  atmosphere  more 
easily  than  negative  electricity. 

From  the  law  of  electric  attraction  (which  is,  that  it  is  inversely 
as  the  square  of  the  distance),  the  discharge  ought  to  fall  first  on  the 
nearest  and  best-conducting  objects,  and,  in  fact,  trees,  elevated 
buildings,  metals,  are  more  particularly  struck  by  the  discharge, 
Hence  it  is  imprudent  to  stand  under  trees  during  a  thunder- 
storm. 

The  effects  of  lightning  are  very  varied,  and  of  the  same  kind  as 
those  of  batteries  (431),  but  of  far  greater  intensity.  The  lightning 
discharge  kills  men  and  animals,  inflames  combustible  matters, 
melts  metals,  breaks  bad  conductors  in  pieces.  When  it  pene- 
trates the  ground  it  melts  the  siliceous  substance  in  its  way,  and 
thus  produces  in  the  direction  of  the  discharge  those  remarkable 
vitrified  tubes  called  fulgurites,  some  of  which  are  as  much  as  twelve 
yards  in  length.  When  it  strikes  bars  of  iron,  it  magnetises  them, 
and  often  inverts  the  poles  of  compass  needles. 

After  the  passage  of  lightning,  a  highly  peculiar  odour  is  some- 
times produced,  like  that  perceived  in  a  room  in  which  an  electrical 
machine  is  being  worked.  This  odour  is  due  to  the  formation  of 
a  peculiar  oxygenised  compound,  to  which  the  name  ozone  has 
been  given ;  this,  we  have  seen,  is  considered  to  be  a  peculiar 
allotropic  modification  of  oxygen. 

Many  persons  have  a  very  lively  fear  of  the  effects  of  the  light- 
ning discharge.  This  fear  would  be  materially  diminished  if  we 
remembered  the  very  small  number  of  persons  who  are  really  killed 
by  lightning.  Arago  has  estimated  the  number  for  France  at  twenty 
in  a  year  ;  this  is,  one  victim  for  two  million  inhabitants ;  which  is 

H  H 


466  Frictional  Electricity.  [444- 

a  far  less  proportion  than  that  of  many  other  accidents  which  do 
not  excite  nearly  so  much  fear. 

445.  Return  shock.— This  is  a  violent  and  sometimes  fatal  shock 
which  men  and  animals  experience,  even  when  at  a  great  distance 
from   the  place   where   the  lightning  discharge  passes.     This   is 
caused  by  the  inductive  action  which  the  thunder-cloud  exerts  on 
bodies  placed  within  the  sphere  of  its  activity.     These  bodies  are 
then,  like  the  ground,  charged  with  the  opposite  electricity  to  that 
of  the  cloud  ;  but  when  the  latter  is  discharged  by  the  recombina- 
tion of  its  electricity  with  that  of  the  ground,  the  induction  ceases, 
and  the  bodies  reverting  rapidly  from  the  electrical  state  to  the 
neutral  state,  the  concussion  in  question  is  produced,  the  return 
shock.   A  more  gradual  decomposition  and  reunion  of  the  electricity 
produces  no  visible  effects  ;  yet  it  appears  that  such  disturbances  of 
the  electrical  equilibrium  are  perceived  by  nervous  persons. 

The  return  shock  is  always  less  violent  than  the  direct  one  ; 
there  is  no  instance  of  its  having  produced  any  inflammation,  yet 
plenty  of  cases  in  which  it  has  killed  both  men  and  animals  ;  in 
such  cases  no  broken  limbs,  wounds,  or  burns,  are  observed. 

The  return  shock  may  be  imitated  by  placing  a  gold  leaf  elec- 
troscope, connected  by  a  wire  with  the  ground,  near  the  prime 
conductor  of  a  strong  electrical  machine  in  action  ;  at  each  spark 
taken  from  the  machine,  the  leaves  suddenly  diverge. 

446.  liig-htning-  conductor. — The  ordinary  form  of  this  instru- 
ment is  an  iron  rod,  through  which  passes  the  electricity  of  the 
ground  attracted  by  the  opposite  electricity  of  the  thunder-clouds. 
It  was  invented  by  Franklin  in  1755. 

There  are  two  principal  parts  in  a  lightning  conductor  :  the  rod 
and  the  conductor.  The  rod  (fig.  371)  is  a  pointed  bar  of  iron, 
fixed  vertically  to  the  roof  of  the  edifice  to  be  protected  ;  it  is  from 
six  to  ten  feet  in  height,  and  its  basal  section  is  about  two  or  three 
inches  in  diameter.  The  conductor  is  a  bar  of  iron  which  descends 
from  the  bottom  of  the  rod  to  the  ground,  which  it  penetrates  to 
some  distance.  As,  in  consequence  of  their  rigidity,  iron  bars 
cannot  always  be  well  adapted  to  the  exterior  of  buildings,  lightning 
conductors  are  best  formed  of  wire  cords,  such  as  are  used  for  rig- 
ging and  for  suspension  bridges.  The  conductor  is  usually  led  into 
a  well,  and  to  connect  it  better  with  the  soil  it  ends  in  two  or  three 
branches.  If  there  is  no  well  in  the  neighbourhood,  a  hole  is  dug 
in  the  soil  to  a  depth  of  six  or  seven  yards,  and  the  foot  of  the  con- 
ductor having  been  introduced,  the  hole  is  filled  with  wood-ashes, 


-446] 


Lightning  Conductor. 


467 


which  conduct  very  well  and  yet  preserve  the  metal  from  oxidation. 
Powdered  coke  does  equally  well. 

As  the  action  of  a  lightning  conductor  depends  on  induction  and 
the  power  of  points  (410),  Franklin,  as  soon  as  he  had  established 
the  identity  of  lightning  and  electricity,  assumed  that  lightning  con- 
ductors withdrew  electricity  from  the  clouds  ;  the  converse  is  the 
case.  When  a  storm-cloud,  positively  electrified,  for  instance,  rises 
in  the  atmosphere,  it  acts  inductively  on  the  earth,  repels  the  posi- 


Fig.  370. 

tive  and  attracts  the  negative  fluid,  which  accumulates  in  b6dies 
placed  on  the  surface  of  the  soil  the  more  abundantly  as  these 
bodies  are  at  a  greater  height.  The  tension  is  then  greatest  on  the 
highest  bodies,  which  are  therefore  most  exposed  to  the  electric 
discharge  ;  but  if  these  bodies  are  provided  with  metallic  points, 
like  the  rods  of  conductors,  the  negative  fluid,  withdrawn  from  the 
soil  by  the  influence  of  the  cloud,  flows  into  the  atmosphere,  and 
neutralises  the  positive  fluid  of  the  cloud.  Hence,  not  only  does  a 
lightning  conductor  tend  to  prevent  the  accumulation  of  electricity 
on  the  surface  of  the  earth,  but  it  also  tends  to  restore  the  clouds  to 
their  natural  state,  both  which  actions  concur  in  preventing  lightning 
discharges.  /The  disengagement  of  electricity  is,  however,  some- 
times so  abundant,  that  the  lightning  conductor  is  inadequate  to 
discharge  the  ground,  and  the  lightning  strikes;  but  the  conductor 

H  H  2 


468 


Fractional  Electricity. 


[446- 


receives  the  discharge,  in  consequence  of  its  greater  conductivity, 
and  the  building  is  preserved. 

It  is   stated  that,  approximately,  a  lightning  conductor  pro- 
tects a  circular  space  around  it,  the  radius  of  which  is  double  its    ( 


Fig.  37 '• 

height.  Thus  a  building,  sixty-four  yards  in  length,  would  be  pre- 
served by  two  rods  eight  yards  in  height,  at  a  distance  of  thirty-two 
yards. 

447.  Aurora  borealis. — The  aurora  borealis,  or  northern  light, 
or  more  properly  polar  aurora,  is  a  remarkable  luminous  phe- 
nomenon which  is  frequently  seen  in  the  atmosphere  at  the  two 
terrestrial  poles,  but  more  especially  at  the  north  pole.  At  the 
close  of  the  day  an  indistinct  light  appears  in  the  horizon  in  the 
direction  of  the  magnetic  meridian.  This  luminosity  gradually 
changes  into  a  regular  arc  of  a  pale  yellow  with  its  concave  side 
turned  towards  the  earth.  Finally,  the  rays  burst  all  over  the  hori- 
zon, passing  necessarily  from  yellow  to  deep  green,  and  to  the  most 
brilliant  purple.  All  these  rays  converge  towards  one  ppint  of  the 
horizon,  which  is  the  prolongation  of  thte  dipping  needle,  and  they 
form  then  a  fragment  of  an  immense  luminous  cupola. 


447] 


Aurora  Borealis. 


469 


When  the  luminous  arc  is  formed  it  often  remains  visible  for 
some  hours  ;  then  the  lustre  diminishes,  the  colours  disappear,  and 
this  brilliant  phenomenon  gradually  diminishes,  or  is  suddenly 
extinguished. 

Numerous  hypotheses  have  been  devised  to  account  for  the 
auroras  boreales.  The  constant  direction  of  their  arc  as  regards  the 
magnetic  meridian,  and  their  action  on  the  magnetic  needle  (394), 
show  that  they  ought  to  be  attributed  to  electric  currents  in  the 


Fig.  372. 

higher  regions  of  the  atmosphere.  This  hypothesis  is  confirmed  by 
the  circumstance  that  during  the  prevalence  of  the  aurora  borealis, 
electric  telegraph  lines  are  spontaneously  affected  in  a  powerful 
but  irregular  manner  ;  needles  are  deflected,  armatures  attracted, 
and  alarums  rung.  This  interference  is  at  times  so  serious,  espe- 
cially in  northern  countries,  that  it  is  necessary  to  suspend  the 
ordinary  transmission  of  messages. 

According  to  M.  de  la  Rive  the  auroras  boreales  are  due  to 
electric  discharges  which  take  place  in  polar  regions  between  the 
positive  electricity  of  the  atmosphere  and  the  negative  electricity  of 
the  terrestrial  globe  ;  electricities  which  themselves  are  separated 
by  the  action  of  the  sun,  principally  in  the  equatorial  regions. 


47°  Frictional  Electricity.  [447- 

In  Chapter  XII.  an  experiment  will  be  described  which  De  la 
Rive  has  devised  in  support  of  this  hypothesis. 

448.  St.  Elmo's  fire. — This  name  is  given  by  sailors  to  the 
luminous  brushes  or  stars  which  sometimes  appear  at  the  tops  of 
masts  and  yards  of  vessels,  and  which  are  sometimes  accompanied 
by  a  cracking  sound,  which  resembles  the  sparks  taken  from  elec- 
trical machines. 

These  luminous  effects  were  known  to  the  ancients.  Pliny 
speaks  of  the  fiery  stars  seen  on  the  ends  of  soldiers'  lances.  When 
they  were  two  in  number  they  were  compared  to  Castor  and  Pollux, 
and  that  was  a  favourable  presage  ;  if  only  one  appeared,  it  was 
likened  to  their  sister  Helena,  which  was  considered  a  bad  omen. 

St.  Elmo's  fire  is  a  simple  case  of  induction.  The  atmospheric 
electricity  acting  on  conductors  decomposes  the  neutral  fluid,  at- 
tracting the  contrary  electricity  ;  which,  from  the  power  of  points, 
being  liberated  at  the  extremities  of  the  masts,  or  by  the  metal  of 
the  lances,  gives  rise  to  the  luminous  brush.  The  same  effect  is 
observed  when,  placing  a  metal  point  on  the  conductor  of  the  elec-  • 
trical  machine,  it  is  made  to  work  in  darkness. 


CHAPTER  VII. 

ELECTRICITY  DUE  TO  CHEMICAL  ACTION.      VOLTAIC  BATTERY. 

449.  Calvani's  experiment. — We  have  already  seen  that  the 
two  most  powerful  sources  of  electricity  are  friction  and  chemical 
combination.  Having  described  the  former,  we  are  now  to  be 
concerned  with  the  latter.  Yet  we  may  premise  that  this  is  not  a 
new  kind  of  electricity,  but  only  another  method  for  its  production 
far  more  abundant  than  friction,  and  leading  to  the  most  remarkable 
effects. 

To  Galvani,  professor  of  anatomy  in  Bologna,  is  due  the  dis- 
covery in  1790  of  these  new  electrical  phenomena,  to  which  he 
was  led  by  a  casual  observation.  It  is  said  that  a  dead  frog  being 
accidentally  suspended  by  a  hook  of  copper  to  the  iron  railings 
of  a  balcony,  was  observed  to  be  violently  contracted  whenever 
the  legs  of  the  animal  came  in  contact  with  the  iron  bars. 

Galvani's  observation  may  be  reproduced  in  the  following  man- 
ner :  the  legs  of  a  recently  killed  frog  are  prepared,  and  suspended 


-450] 


Galvani's  Experiment. 


471 


to  a  copper  hook,  which  passes  between  the  vertebral  column, 
and  the  nerve  filaments  on  each  side  of  it.  If  then  the  copper 
support  and  the  legs  are  momentarily  connected  by  a  plate  of 
zinc  z,  at  each  contact  a 
smart  contraction  of  the 
muscles  ensues  (fig.  373). 

Galvani  had  some  time 
before,  observed  that  the 
electricity  of  machines 
produced  in  dead  frogs 
analogous  contractions, 
and  he  attributed  the 
phenomena  first  described 
to  an  electricity  inherent 
in  the  animal.  He  as- 
sumed that  this  electri- 
city, which  he  called  vital 
fluid,  passed  from  the 
nerves  to  the  muscles  by 
the  metallic  arc,  and  was 
thus  the  cause  of  contrac- 
tion. This  theory  met 
with  great  support,  es- 
pecially among  physiolo- 
gists, but  it  was  not  without  opponents.  The  most  considerable 
of  these  was  Alexander  Volta,  professor  of  physics  in  Pavia. 

450.  Volta's  fundamental  experiment. — Galvani's  attention 
had  been  exclusively  devoted  to  the  nerves  and  muscles  of  the  frog ; 
Volta's  was  directed  upon  the  connecting  metal.  Resting  on  the 
observation,  which  Galvani  had  also  made,  that  the  contraction  is 
more  energetic  when  the  connecting  arc  is  composed  of  two  metals 
than  when  there  is  only  one,  Volta  attributed  to  the  metals  the 
active  part  in  the  phenomenon  of  contraction.  He  assumed  that 
the  disengagement  of  electricity  was  due  to  their  contact,  and  that 
the  animal  parts  only  officiated  as  conductors,  and  at  the  same 
time  as  a  very  sensitive  electroscope. 

By  means  of  the  then  recently  invented  electroscope,  Volta  de- 
vised several  modes  of  showing  the  disengagement  of  electricity  on 
the  contact  of  metals,  of  which  the  following  is  the  easiest  to  per- 
form : 

The  moistened  finger  being  placed  on  the  upper  plate  of  a  con- 


Fig.  373- 


472 


Voltaic  Electricity. 


[450- 


clensing  electroscope  (fig.  362,  p.  453),  the  lower  plate  is  touched  with 
a  plate  of  copper,  c,  soldered  to  a  plate  of  zinc,  z,  which  is  held  in 
the  other  hand.  On  breaking  the  connection  and  lifting  the  upper 
plate  (hg.  363),  the  gold  leaves  diverge,  and,  as  may  be  proved,  with 
negative  electricity.  Hence,  when  soldered  together,  the  copper 
is  charged  with  negative  electricity,  and  the  zinc  with  positive  elec- 
tricity. The  electricity  could  not  be  due  either  to  friction  or  pres- 
sure ;  for  if  the  condenser  plate,  which  is  of  copper,  is  touched  with 
the  zinc  plate,  z,  the  copper  plate  to  which  it  is  soldered  being  held 
in  the  hand,  no  trace  of  electricity  is  observed. 

A  memorable  controversy  arose  between  Galvani  and  Volta. 
The  latter  was  led  to  give  greater  extension  to  his  contact  theory, 
and  propounded  the  principle,  that  when  two  heterogeneous  sub- 
stances are.  placed  in  contact,  one  of  them  always  assumes  the  posi- 
tive and  the  other  the  negative  electrical  condition.  In  this  form 
Yolta's  theory  obtained  the  assent  of  the  principal  philosophers  of 
his  time. 


Fig.  374- 
451.  Voltaic  pile. — Reasoning  from  this  theory  of  contact,  Volta 


-452]  Voltaic  Pile.  473 

was  led,  in  1800,  to  the  invention  of  the  marvellous  instrument  which 
immortalised  him,  and  which  is  known  to  this  day  as  the  Voltaic 
pile.  Wishing  to  multiply  the  points  of  contact,  and  to  collect  the 
electricities  produced  by  each,  Volta  arranged,  as  represented  in 
figure  374,  a  disc  of  zinc,  a  disc  of  copper,  then  a  round  piece  of 
cloth  moistened  with  acidulated  water,  then  again  a  disc  of  zinc,  a 
disc  of  copper,  a  piece  of  cloth,  and  so  forth,  great  care  being  taken 
always  to  preserve  the  same  order.  What  was  to  be  expected  from 
such  a  combination  ?  Arago  says,  '  I  do  not  hesitate  to  assert,  that 
this  mass  so  inert  in  appearance,  this  pile  of  so  many  couples 
of  metal  separated  by  a  little  liquid,  is,  as  regards  the  singularity  of 
its  effects,  the  most  remarkable  instrument  which  has  ever  been  in- 
vented, without  even  excepting  the  telescope  and  the  steam  engine.' 

On  Volta's  view  the  union  of  one  zinc  and  one  copper  forms  a 
couple  ;  in  the  above  figure  twenty  couples  are  superposed,  separated 
from  each  other  by  pieces  of  cloth,  and  all  arranged  in  the  same 
order,  so  that  one  extremity  terminates  in  a  disc  of  copper,  and  the 
other  in  a  disc  of  zinc.  Since  its  invention  it  has  been  greatly 
modified  ;  but  the  general  name  of  pile  has  been  retained  for  all 
apparatus  of  the  same  kind,  and  the  electricity  furnished  by  piles  is 
spoken  of  as  voltaic  electricity. 

452.  Disengagement  of  electricity  in  chemical  actions. — 
The  contact  theory  which  Volta  had  propounded,  and  in  which  he  ex- 
plained the  action  of  the  pile,  soon  encountered  objectors.  Fabroni, 
a  countryman  of  Volta,  having  observed  that  in  the  pile  the  discs 
of  zinc  became  oxidised  in  contact  with  the  acidulated  water, 
thought  that  this  oxidation  was  the  principal  cause  of  the  disen- 
gagement of  electricity.  In  England  Wollaston  soon  advanced  the 
same  opinion,  and  Davy  supported  it  by  many  ingenious  experi- 
ments. 

It  is  true  that  in  the  fundamental  experiment  of  the  contact 
theory  (451)  Volta  obtained  signs  of  electricity.  But  M.  de  la  Rive 
has  shown,  that  if  the  zinc  be  held  in  a  wooden  clamp,  all  signs  of 
electricity  disappear,  and  that  the  same  is  the  case  if  the  zinc  be 
placed  in  gases,  such  as  hydrogen  or  nitrogen,  which  exert  upon  it 
no  chemical  action.  De  la  Rive  has  accordingly  concluded,  that  in 
Volta's  original  experiment  the  disengagement  of  electricity  is  due 
to  the  chemical  actions  which  result  from  the  perspiration  and  from 
the  oxygen  of  the  atmosphere. 

By  a  variety  of  analogous  experiments  it  may  be  shown,  that  all 
chemical  actions  are  accompanied  by  a  disturbance  of  the  electrical 


474  Voltaic  Electricity.  [452- 

equilibrium.  This  is  the  case  whether  the  substances  concerned  in 
the  action  are  in  the  solid,  liquid,  or  gaseous  state,  though  of  all 
chemical  actions  those  between  metals  and  liquids  are  the  most 
productive  of  electricity.  All  the  various  resultant  effects  may  be 
explained  on  the  general  principle,  that  when  a  liquid  acts  che- 
mically on  a  metal  the  liquid  assumes  the  positive  electrical,  and  the 
metal  the  negative  electrical  condition. 

Hence  we  arrive  at  a  theory  of  the  origin  of  electricity  in  the 
voltaic  pile  which  will  be  best  illustrated  by  reference  to  the  follow- 
ing simple  experiment. 

453.  Current  electricity. — When  a  plate  of  zinc,  and  a  plate  of 
copper  are  partially  immersed  in  dilute  sulphuric  acid,  by  means  of 
delicate  electroscopic  arrangements  it  may  be  shown,  that  the  zinc 
plate  possesses  a  feeble  charge  of  negative  and  the  copper  plate  a 
feeble  charge  of  positive  electricity.     At  the  same  time  there  is  a 
slight  disengagement  of  hydrogen  gas  from  the  surface  of  the  zinc. 
If  now  the  plates  be'  placed  in  direct  contact,  or,  more  conveniently, 
be  connected  by  means  of  a  metal  wire,  the  chemical  action  in- 
creases, but  the  hydrogen  is  now  disengaged  from  the  surface  of 

the  copper  (fig.  375)  ;  and  if  the  connect- 
ing wire  be  examined,  it  will  be  found  to 
possess  the  remarkable  properties  charac- 
teristic of  the  discharge  of  opposite  elec- 
tricities. So  long  as  the  metals  remain  in 
the  liquid,  the  opposite  electrical  con- 
ditions of  the  two  plates  discharge  them- 
selves by  means  of  the  wire,  but  are  in- 
'stantaneously  restored,  and  as  rapidly 
discharged ;  and  as  these  successive 
charges  and  discharges  take  place  at  such 
Flg-  375>  infinitely  small  intervals  of  time  that  they 

may  be  considered  continuous,  the  wire  is  said  to  be  traversed  by 
an  electric  or  voltaic  current.  The  direction  of  this  current  in  the 
connecting  wire  is  assumed  to  be  from  the  copper  to  the  zinc  ;  or,  in 
other  words,  this  is  the  direction  in  which  the  positive  electricity  is 
supposed  to  flow,  the  direction  of  the  negative  current  in  the  wire 
being  from  the  zinc  to  the  copper.  But  the  existence  of  this 
current  is  purely  hypothetical,  and  must  not  be  taken  as  more  than 
a  convenient  mode  of  explaining  the  phenomena  developed  in  the 
wire. 

454.  Voltaic  couple.     Electromotive   series. — The  arrange- 


-454]  .        Electromotive  Series.  475 

ment  just  described,  consisting  of  two  metals  in  metallic  contact, 
and  a  conducting  liquid  in  which  they  are  placed,  constitutes  a 
simple  voltaic  element  or  couple.  So  long  as  the  metals  are  not 
in  contact,  the  couple  is  said  to  be  open,  and  when  connected  it  is 
closed. 

For  the  production  of  a  voltaic  current  it  is  not  necessary  that 
one  of  the  metals  be  .unaffected  by  the  liquid,  but  merely  that  the 
chemical  action  upon  the  one  be  greater  than  upon  the  other.  The 
metal  which  is  most  attacked  is  called  \^\&.  positive  or  generating 
plate,  and  that  which  is  leq.st  attacked  the  negative  or  collecting 
plate.  The  positive  metal  determines  the  direction  of  the  current, 
which  proceeds  in  the  liquid  from  the  positive  to  the  negative  plate, 
and  out  of  the  liquid  through  the  connecting  wire  from  the  negative 
to  the  positive  plate. 

In  speaking  of  the  direction  of  the  current,  the  positive  current 
is  always  understood  ;  to  avoid  confusion,  the  existence  of  the  current 
in  the  opposite  direction,  the  negative  current,  is  tacitly  ignored. 

As  a  voltaic  current  is  produced  whenever  two  metals  are  placed 
in  metallic  contact  in  a  liquid  which  acts  more  powerfully  upon  one 
than  upon  the  other,  there  is  great  choice  in  the  mode  of  producing 
such  currents.  In  reference  to  their  electrical  deportment,  the 
metals  have  been  arranged  in  what  is  called  an  electromotive  series^ 
in  which  the  most  electropositive  are  at  one  end,  and  the  most 
electronegative  at  the  other.  Hence  when  any  two  of  these  are 
placed  in  contact  in  dilute  acid,  the  current  in  the  connecting  wire 
proceeds  from  the  one  lower  in  the  list  to  the  one  higher.  The 
principal  metals  are  as  follows  : — 

Zinc  Nickel  Gold 

Lead  Copper  Platinum 

Iron  Silver  Graphite. 

Thus  iron  placed  in  dilute  sulphuric  acid  is  electronegative 
towards  zinc,  but  is  electropositive  towards  copper;  copper  in  turn 
is  electro-negative  towards  iron  and  zinc,  but  is  electropositive 
towards  silver,  platinum,  or  graphite. 

The  force  produced  by  the  difference  in  chemical  action  on  two 
metals  in  a  liquid  is  called  the  electromotive  force  ;  it  is  greater  in 
proportion  to  the  distance  of  the  two  metals  from  one  another  in  the 
series.  That  is  to  say,  it  is  greater,  the  greater  the  difference 
between  the  chemical  action  upon  the  two  metals  immersed.  Thus 


476  Voltaic  Electricity.  [454- 

the  electromotive  force  between  zinc  and  platinum  is  greater  than 
that  between  zinc  and  iron,  or  between  zinc  and  copper. 

455.  Poles   and  electrodes. — If  the  wire  connecting  the  two 
terminal  plates  of  a  /oltaic  couple  be  cut,  it  is  clear,  from  what  has 
been  said  about  the  origin  and  direction  of  the  current,  that  positive 
electricity  will  tend  to  accumulate  at  the  end  of  the  wire  attached 
to  the  copper  or  negative  plate,  and  negative  electricity  on  the  wire 
attached  to  the  zinc  or  positive  plate.     These  terminals  have  been 
called  the  poles  of  the  battery.      For  experimental  purposes,  more 
especially  in  the  decomposition  of  salts,  plates   of  platinum  are 
attached  to  the  ends  of  the  wires.     Instead  of  the  term  poles  the 
word  electrode  (/jXwrpr/v  and  6ody,  a  way)  is  now  commonly  used  ; 
for  these  are  the  ways  through  which  the  respective  electricities 
emerge.     It  is  important  not  to  confound  the  positive  plate  with  the 
positive  pole  or  electrode.     The  positive  electrode  is  that  connected 
with  the  negative  plate,  while  the  negative  electrode  is  connected 
with  the  positive  plate. 

456.  Voltaic  battery. — When  a  series  of  voltaic  elements  or 
pairs  are  arranged  in  such  a  manner  that  the  zinc  of  one  element  is 
connected  with  the  copper  of  another ;    the  zinc  of  this  with  the 
copper  of  another,  and  so  on,  such  an  arrangement  is  called  a  vol- 
taic battery  ;  and  by  its  means  the  effects  produced  by  a  single 
element  are  capable  of  being  very  greatly  increased. 

The  earliest  of  these  arrangements  was  the  voltaic  pile  devised 
by  Volta  himself. 

It  will  be  readily  seen  that  it  is  merely  a  series  of  simple  voltaic 
couples,  the  moistened  disc  acting  as  the  liquid,  and  that  the 
terminal  zinc  is  the  negative  and  the  terminal  copper  the  positive 
pole.  From  the  mode  of  its  arrangement,  and  from  its  discoverer, 
the  apparatus  is  known  as  the  voltaic  pile,  a  term  applied  to  all 
apparatus  of  this  kind  for  accumulating  the  effects  of  dynamical 
electricity. 

The  distribution  of  electricity  in  the  pile  varies  according  as  it  is 
in  connection  with  the  ground  by  one  of  its  extremities,  or  as  it  is 
insulated  by  being  placed  on  a  non-conducting  cake  of  resin  or 
glass. 

In  the  former  case,  the  end  in  contact  with  the  ground  is  neutral, 
and  the  rest  of  the  apparatus  only  contains  one  kind  of  electricity  ; 
this  is  negative,  if  a  copper  disc  is  in  contact  with  the  ground,  and 
positive  if  it  is  a  zinc  disc. 

In  the  insulated  pile  the  electricity  is  not  uniformly  distributed. 


-456] 


-    Voltaic  Battery. 


477 


By  means  of  the  proof-plane  and  the  electroscope  it  may  be  de- 
monstrated that  the  middle  part  is  in  a  neutral  state,  and  that' one- 
half  is  charged  with  positive  and  the  other  with  negative  electricity, 
the  tension  increasing  from  the  middle  to  the  ends.  The  half 
terminated  by  a  zinc  is  charged  with  negative  electricity,  and  that 
by  a  copper  with  positive  electricity.  The  effects  of  the  pile  will 
be  discussed  in  other  places. 

The  original  form  of  the  voltaic  pile — for  it  possesses  now  only 
an  historical  interest — has  a  great  many  inconveniences  ;  among 
these  is  the  fact,  that  the  weight  of  the  discs  of  zinc  and  copper  is 
so  great  that  it  presses  out  the  acidulated  liquid  from  the  discs,  and 
the  electrical  action  is  soon  weakened.  It  has  received  a  great 
many  improvements,  the  principal  object  of  which  has  been  to 
facilitate  manipulation,  and  to  produce  greater  electromotive  force. 

One  of  the  earliest  of  these  modifications  was  the  crown  of  cups, 
or  couronne  des  tasses,  invented  by  Volta  himself :  an  improved 
form  of  this  is  known  as  Wollaston's  battery  (fig.  378). 

Fig.  376  gives  a  vertical  section  of  two  consecutive  Wollaston's 
elements.  The  acidulated  water  is  contained  in  glass  vessels,  B  B 


Fig.  376. 


Fig.  377- 


in  each  of  which  is  a  couple.  Fig.  377  represents  the  arrangement 
of  one  of  these  couples  :  it  consists  of  a  thick  sheet  of  zinc,  Z,  and  a 
strip  of  copper,  0,  by  which  it  can  be  connected  with  the  next 
couple.  A  plate  of  copper,  C  C,  is  bent  so  as  to  sun  ound  the  plate 
of  zinc  without  touching,  contact  being  prevented  by  small  pieces  of 


47* 


Voltaic  Electricity. 


[456- 


cork.    The  plate,  C,  is  provided  with  a  copper  tongue,  o',  which  is 
soldered  to  the  zinc  of  the  next  couple  and  so  forth. 

Fig.   378  represents  a   pile  of  sixteen  couples  united   in   two 
parallel  series  of  eight  each.     All  these  couples  are  fixed  to  a  cross 


Fig.  378- 

frame  of  wood,  by  which  they  can  be  raised  or  lowered  at  pleasure. 
When  the  battery  is  not  wanted,  the  couples  are  lifted  out  of  the 
liquid.  The  water  in  these  vessels  is  usually  acidulated  with  ^ 
sulphuric  and  ^  of  nitric  acid. 

457.  Enfeeblement  of  tne  current  in  batteries.  Secondary 
currents. — The  batteries  already  described,  Volta's  and  Wollaston's, 
which  consist  essentially  of  two  metals  and  one  liquid,  labour  under 
the  objection  that  the  currents  produced  rapidly  diminish  in  inten- 
sity. 

This  is  principally  due  to  three  causes  ;  the  first  is  the  decrease 
in  the  chemical  action  owing  to  the  neutralisation  of  the  sulphuric 
acid  by  its  combination  with  the  zinc.  This  is  a  necessary  action, 
for  upon  it  depends  the  current ;  it  therefore  occurs  in  all  batteries, 
and  is  without  remedy,  except  by  replacement  of  the  acid  and  zinc. 
The  second  is  due  to  what  is  called  local  action  ;  that  is,  the  pro- 
duction of  small  closed  currents  in  the  active  metal,  from  the  im- 
purities it  contains.  These  local  currents  rapidly  wear  away  the 
active  plate,  without  contributing  anything  to  the  general  current. 


-459] 


Danieirs  Battery. 


479 


They  are  remedied  by  amalgamating  the  zinc  with  mercury,  by 
which  chemical  action  is  prevented  until  the  circuit  is  closed.  The 
third  arises  from  secondary  currents.  These  are  currents  which  are 
produced  in  the  battery  in  a  contrary  direction  to  the  principal  cur- 
rent, and  which  destroy  it  either  totally  or  partially.  In  the  funda- 
mental experiment  (fig.  367),  when  the  current  is  closed,  sulphate 
of  zinc  is  formed,  which  dissolves  in  the  liquid,  and  at  the  same 
time  a  layer  of  hydrogen  gas  is  gradually  deposited  on  the  surface 
of  the  copper  plate.  Now  it  has  been  found,  that  the  hydrogen  de- 
posited in  this  manner  on  metallic  surfaces  acts  far  more  energeti- 
cally than  ordinary  hydrogen.  In  virtue  of  this  increased  activity 
it  gradually  reduces  some  of  the  sulphate  of  zinc  formed,  and  a 
layer  of  metallic  zinc,  is  formed  upon  the  copper  ;  hence,  instead  of 
having  two  different  metals  unequally  attacked,  the  two  metals  be- 
come gradually  less  different,  and,  consequently,  in  the  wire  there 
are  two  currents  tending  to  become  equal  ;  the  total  effect,  and  the 
current  really  observed,  become  weaker  and  weaker. 

458.  Constant  batteries.      Banieil's. — The  serious  objections 
to  the  use  of-  what  are  called  single  fluid  elements  has  led  to  their 
abandonment,  and  they  are  now  replaced  by  two  fluid  elements, 
which  are  both  more  constant  and  more  powerful.    They  have  been 
replaced  by  batteries  with  two  liquids,  which  are  called  constant 
batteries,  because  their  action  is  without  material  alteration  for  a 
considerable  period  of  time.     The  es- 
sential  point    to    be    attended   to   in 

securing  a  constant  current  is  to  pre- 
vent the  polarisation  of  the  inactive 
metal  ;  in  other  words,  to  hinder  any 
permanent  deposition  of  hydrogen  on 
its  surface.  This  is  effected  by  plac- 
ing the  inactive  metal  in  a  liquid  upon 
which  the  deposited  hydrogen  can  act 
chemically. 

459.  Daniell's  battery. — This  was 
the  first  form  of  the  constant  battery, 
and  was  invented  by  Daniell   in  the 
year  1 836.     As  regards  the  constancy 
of  its  action,  it  is  still  the  best  of  all 
constant   batteries.     Fig.   379    repre- 
sents a  single  element.     A  glass  or  porcelain  vessel,  V,  contains  a 
saturated  solution  of  sulphate  of  copper,  in  which  is  immersed  a 


4^0 


Voltaic  Electricity. 


[459- 


copper  cylinder,  C,  open  at  both  ends,  and  perforated  by  holes. 
At  the  upper  part  of  this  cylinder  there  is  an  annular  shelf,  G, 
also  perforated  by  small  holes,  and  below  the  level  of  the  solution : 
this  is  intended  to  support  crystals  of  sulphate  of  copper  to  replace 
that  decomposed  as  the  electrical  action  proceeds.  Inside  the 
cylinder  is  a  thin  porous  vessel,  P,  of  unglazed  earthenware.  This 
contains  either  a  solution  of  common  salt  or  dilute  sulphuric  acid> 
in  which  is  placed  the  cylinder  of  amalgamated  zinc,  Z.  Two  thin 
strips  of  copper,  p  and  «,  fixed  by  binding  screws  to  the  copper 
and  to  the  zinc,  serve  for  connecting  the  elements  in  series. 

When  a  DanielPs  element  is  closed,  the  hydrogen  resulting  from 


Fig.  380. 

the  action  of  the  dilute  acid  on  the  zinc  is  liberated  on  the  surface 
of  the  copper  plate,  but  meets  there  the  sulphate  of  copper,  which 
is  reduced,  forming  sulphuric  acid  and  metallic  copper  which  is 
deposited  on  the  surface  of  the  copper  plate.  In  this  way  the  sul- 
phate of  copper  in  the  solution  is  taken  up,  and  if  it  were  all  con- 
sumed, hydrogen  would  be  deposited  on  the  copper,  and  the  current 
would  lose  its  constancy.  This  is  prevented  by  the  crystals  of  sul- 
phate of  copper  which  keep  the  solution  saturated.  The  sulphuric 
acid  produced  by  the  decomposition  of  the  sulphate  permeates  the 
porous  cylinder,  and  tends  to  replace  the  acid  used  up  by  its  action 
on  the  zinc  ;  and  as  the  quantity  of  sulphuric  acid  formed  in  the 
solution  of  sulphate  of  copper  is  regular,  and  proportional  to  the 
acid  used  in  dissolving  the  zinc,  the  action  of  this  acid  on  the  zinc 
is  regular  also,  and  thus  a  constant  current  is  produced. 


-460] 


BunseiJs  Battery. 


481 


Fig.  380  represents  a  series  of  three  Daniell's  elements  of  a 
somewhat  different  pattern.  Here  the  zinc  of  one  is  connected 
with  the  copper  of  the  next  by  a  copper  strip.  Instead  of  placing 
the  crystals  of  sulphate  of  copper  on  a  shelf  in  the  copper  plate, 
they  are  contained  in  glass  flasks,  the  necks  of  which  are  immersed 
in  a  solution  of  sulphate  of  copper.  This  form  of  element  is  exten- 
sively used  in  the  French  telegraphs. 

460.  Bunsen's  battery. — Bunsen's  battery,  also  known  as  the 
sine  carbon  battery,  was  invented  in  1 843  ;  it  is  nothing  more  than 
Daniell's  batteiy,  in  which  nitric  acid  is  substituted  for  solution  of 
sulphate  of  copper,  and  in  which  copper  is  replaced  by  a  cylinder 


Fig.  381. 

of  carbon.  This  is  made  either  of  the  graphitoidal  carbon  depo- 
sited in  gas  retorts,  or  by  calcining  in  an  iron  mould  an  intimate 
mixture  of  coke  and  bituminous  coal,  finely  powdered  and  strongly 
compressed.  Both  these  modifications  of  carbon  are  good  con- 
ductors. Each  element  consists  of  the  following  parts  :  i.  a  vessel, 
F  (fig.  381),  either  of  stoneware  or  of  glass,  containing,  as  in 
Daniell's,  dilute  sulphuric  acid  ;  2.  a  hollow  cylinder,  Z,  of  amalga- 
mated zinc  ;  3.  a  porous  vessel,  V,  in  which  is  ordinary  nitric  acid  ; 
4.  a  cylinder  of  carbon,  C,  prepared  in  the  above  manner.  In  the 
vessel,  F,  the  zinc  is  first  placed,  and  in  it  the  carbon  as  seen  in  P. 
To  the  carbon  is  fixed  a  binding  screw,  m  (fig.  382),  to  which  a 
copper  wire  is  attached,  forming  the  positive  pole.  The  zinc  is 
provided  with  a  similar  binding  screw,  #,  and  wire,  which  is  thus 
the  negative  pole. 

In  Bunsen's  battery  the  hydrogen  resulting  from  the  action  is 

I  I 


482 


Voltaic  Electricity. 


[460- 


liberated  on  the  surface  of  the  carbon.  This  being  surrounded  by 
nitric  acid,  the  hydrogen  decomposes  this  acid,  forming  water  and 
hyponitrous  acid,  which  dissolves,  or  is  subsequently  disengaged  as 
nitrous  fumes.  And,  though  the  hydrogen  is  most  completely  got 
rid  of  by  the  decomposition  of  the  nitric  acid,  the  production  of 
these  nitrous  fumes  is  very  noxious. 

The  elements  are  arranged  to  form  a  battery  (fig.  382)  by  con- 
necting each  carbon  to  the  zinc  of  the  following  one  by  means  of  the 


Fig.  382. 

clamps,  mn,  and  a  strip  of  copper,  c,  represented  in  the  top  of  the 
figure.  The  copper  is  pressed  at  one  end  between  the  carbon  and 
the  clamp,  and  at  the  other  it  is  soldered  to  the  clamp,  n,  which  is 
fitted  on  the  zinc  of  the  following  element,  and  so  forth.  The  clamp 
of  the  first  carbon  and  that  of  the  last  zinc  are  alone  provided  with 
binding  screws,  to  which  are  attached  the  wires. 

461.  Xieclan  che's  battery. — Each  element  of  this  battery  con- 
sists of  a  rod  of  carbon  placed  in  a  porous  pot  which  is  then  tightly 
packed  with  a  mixture  of  pyrolusite  (peroxide  of  manganese)  and 
coke.  The  porous  pot  is  contained  in  an  outer  vessel  in  which  is 
the  electropositive  metal  zinc.  The  exciting  liquid  is  a  solution 
of  sal-ammoniac.  This  battery,  from  its  simplicity,  its  constancy, 
combined  with  considerable  electromotive  force  is  coming  into 
use  for  telegraphs,  and  for  alarums  in  private  houses. 


-462] 


Effects  of  the  Battery. 


483 


CHAPTER   VIII. 

EFFECTS   OF  THE   BATTERY. 

462.  Physiological  effects. — The  remarkable  phenomena  of  the 
voltaic  battery  may  be  classed  under  the  heads  physiological, 
chemical,  mechanical,  and  physical  effects  ;  and  these  latter  may 
be  again  subdivided  into  the  thermal,  luminous,  and  magnetic 


Fig.  383- 

effects.  All  are  due  to  the  recombination  of  the  opposite  electri- 
cities like  those  of  the  electrical  machine  ;  but  they  are  far  more 
remarkable  and  more  energetic,  owing  to  the  continuity  of  their 
action.  To  produce  them  the  body  experimented  upon  must  be 
connected  on  the  one  side  with  the  positive  and  on  the  other  with 
the  negative  pole. 

I  I  2 


484  •  Voltaic  Electricity.  [462- 

The  physiological  effects  consist  of  shocks  and  violent  contrac- 
tions which  the  current  produces  in  the  muscles,  not  only  of  living, 
but  of  dead  animals,  as  has  been  seen  in  Galvani's  experiment  with 
the  frog. 

When  the  electrodes  of  a  strong  battery  are  held  in  the  two 
hands  a  violent  shock  is  felt,  resembling  that  of  a  Leyden  jar,  espe- 
cially if  the  hands  are  moistened  with  acidulated  or  saline  water, 
which  increases  the  conductivity.  The  shock  is  more  violent  in 
proportion  to  the  number  of  elements  used  ;  with  a  Bunsen's- 
battery  of  50  to  60  couples  the  shock  is  very  strong,  with  150  or  200 
couples  it  is  unbearable,  and  even  dangerous  when  continued.  It 
is  less  perceptible  in  the  fore  part  of  the  arms  than  the  shock  of  the 
Leyden  jar,  and  when  transmitted  through  a  chain  of  several  per- 
sons, it  is  generally  only  felt  by  those  nearest  the  poles. 

The  shock,  as  in  the  case  of  the  Leyden  jar,  is  due  to  the  recom- 
position  of  the  two  electricities  ;  with  this  difference,  that  with  the 
Leyden  jar  the  discharge  being  instantaneous,  the  resultant  shock 
is  so  also  ;  while  in  the  latter  case,  as  the  battery  is  immediately 
recharged  after  each  discharge,  the  shocks  succeed  each  other  with 
rapidity. 

John  Aldina,  a  nephew  of  Galvani,  was  the  first  to  study  the 
action  of  the  battery  on  dead  animals.  He  came  to  Paris  at  the 
beginning  of  the  present  century,  and  repeated  on  a  large  scale 
several  of  his  experiments  at  the  veterinary  school  of  Alfort  near 
Paris. 

463.  Thermal  effects. — When  a  voltaic  current  is  passed  through 
a  metallic  wire  the  same  effects  are  produced  as  by  the  discharge  of 
an  electric  battery  ;  the  wire  becomes  heated  and  even  incandescent 
if  it  is  very  short  and  thin.  With  a  powerful  battery  all  metals  are 
melted,  even  iridium  and  platinum,  the  least  fusible  of  metals. 
Carbon  is  the  only  body  which  hitherto  has  not  been  fused  by  it. 
M.  Despretz,  however,  with  a  battery  composed  of  600  Bunsen's 
elements  joined  in  six  series,  has  raised  rods  of  very  pure  carbon  to 
such  a  temperature  that  they  were  softened  and  could  be  welded 
together,  indicating  an  incipient  fusion. 

A  battery  of  thirty  to  forty  Bunsen's  elements  is  sufficient  to 
melt  and  volatilise  fine  wires  of  lead,  tin,  zinc,  copper,  gold,  silver, 
iron,  and  even  platinum,  with  differently  coloured  sparks.  Iron 
and  platinum  burn  with  a  brilliant  white  light  ;  lead  with  a  purple 
light  ;  the  light  of  tin  and  of  gold  is  bluish  white  ;  the  light  of  zinc 
is  a  mixture  of  white  and  gold ;  finally,  copper  and  silver  give  a 


-464] 


Luminous  Effects. 


485 


green  light.     The  heat  tig  effects  of  the  voltaic  current  are  used  in 
firing  mines  for  military  purposes  and  for  blasting  operations. 

464.  Luminous  effects. — In  closing  a  voltaic  battery  a  spark  is 
obtained  at  the  point  of  contact,  which  is  frequently  of  great  bril- 
liance. A  similar  spark  is  also  perceived  on  breaking  contact. 
These  luminous  effects  are  obtained  when  the  battery  is  sufficiently 
powerful,  by  bringing  the  two  electrodes  very  nearly  in  contact  ;  a 


Fig.  384- 

succession  of  bright  sparks  springs  across  the  interval,  which 
follow  each  other  with  such  rapidity  as  to  produce  a  continuous 
light.  With  eight  or  ten  of  Grove's  elements  brilliant  luminous 


486  Voltaic  Electricity.  [464- 

sparks  are  obtained  by  connecting  one  terminal  of  the  battery  with 
a  file,  and  moving  its  point  along  the  teeth  of  another  file  connected 
with  the  other  terminal. 

The  most  beautiful  effect  of  the  electric  light  is  obtained  when 
two  pencils  of  charcoal  are  connected  with  the  terminals  of  a 
powerful  battery  ih  the  manner  represented  in  fig.  384.  The  two 
charcoals  being  placed  in  contact  the  current  passes,  and  their  ends 
soon  become  incandescent.  If  they  are  then  removed  to  a  distance  of 
about  the  tenth  of  an  inch,  according  to  the  intensity  of  the  cur- 
rent, a  luminous  arc  extends  between  the  two  points,  which  has  an 
exceedingly  brilliant  lustre,  and  is  called  the  voltaic  arc. 

The  length  of  this  arc  varies  with  the  force  of  the  current.  In 
air  it  may  exceed  two  inches  with  a  battery  of  600  elements.  If  the 
charcoal  attached  to  the  positive  pole  be  examined,  it  will  be  found 
to  have  become  hollowed,  and  worn  away,  while  the  negative 
charcoal  has  increased.  It  thus  seems  that  the  carbon  is  trans- 
ported from  the  positive  to  the  negative  pole,  and  that  this  is  the 
manner  in  which  the  transmission  of  the  electricity  between  the 
two  poles  is  effected. 

The  intensity  of  the  electric  light  is  very  great.  Bunsen,  in  ex- 
perimenting with  forty-eight  couples,  and  removing  the  charcoals 
to  a  distance  of  a  quarter  of  an  inch,  found  that  the  intensity  of 
the  electric  light  is  equal  to  that  of  572  candles. 

Too  great  precautions  cannot  be  taken  against  the  effect  of  the 
electric  light  when  they  attain  a  certain  intensity.  The  light  of 
100  couples,  he  says,  may  produce  very  painful  affections  of  the 
eyes.  With  600,  a  single  moment's  exposure  to  the  light  is  suffi- 
cient to  produce  very  violet  headaches  and  pains  in  the  eye,  and 
the  whole  frame  is  affected  as  by  a  powerful  sunstroke. 

Attempts  have  been  made  to  apply  the  electric  light  to  the  illu- 
mination of  rooms  and  even  of  streets  ;  but  partly  the  cost,  and 
partly  the  difficulty  of  producing  with  it  a  uniform  illumination, 
inasmuch  as  the  shadows  are  thrown  into  too  sharp  relief,  have 
hitherto  been  great  obstacles  to  its  use.  Yet  it  is  advantageously 
applied  in  special  cases,  such  as  the  photo-electric  microscope, 
illuminations  in  theatres,  &c.  Fig.  384  represents  an  arrange- 
ment for  public  illumination.  On  a  convenient  support,  an  electric 
lamp  is  placed  ;  this  is  a  mechanism  which  is  worked  by  the 
current  from  a  powerful  battery,  and  which  keeps  the  carbon  poles 
at  a  suitable  distance,  a  condition  necessary  for  the  permanence 
and  steadiness  of  the  light. 


-465] 


Decomposition  of  Water. 


487 


DECOMPOSITION   OF  WATER. 

465.  Chemical  effects. — These  are  among  the  most  important 
of  all  the  actions,  either  of  the  simple  or  compound  circuit.  They 
consist  of  the  separation  and  transport  of  the  elements  of  the 
bodies  traversed  by  the  current.  The  first  decomposition  effected 
by  the  battery  was  that  of  water,  obtained  in  1800  by  Carlisle  and 
Nicholson  by  means  of  a  voltaic  pile.  Water  is  rapidly  decomposed 
by  four  or  five  Bunsen's  cells  ;  the  apparatus  (fig.  385)  is  very  con- 


Fig. 385- 

venient  for  the  purpose.  It  consists  of  a  glass  vessel  fixed  on  a 
wooden  base.  In  the  bottom  of  the  vessel  two  platinum  electrodes 
are  fitted,  communicating  by  means  of  copper  wires  with  the 
binding  screws,  a  and  b.  The  vessel  is  filled  with  water  to  which 
some  sulphuric  acid  has  been  added  to  increase  its  conductivity, 
for  pure  water  is  a  very  imperfect  conductor  ;  two  glass  tubes  filled 
with  water  are  inverted  over  the  electrodes,  and,  on  interposing  the 
apparatus  in  the  circuit  of  a  battery,  decomposition  is  rapidly  set 
up,  and  gas  bubbles  rise  from  the  surface  of  each  pole.  The  volume 
of  gas  liberated  at  the  negative  pole  is  about  double  that  at  the 
positive,  and  on  examination  the  former  gas  is  found  to  be  hydrogen 
and  the  latter  gas  oxygen.  This  experiment  accordingly  gives  at 
once  the  qualitative  and  quantitative  analysis  of  water ;  for  it  shows 
that  its  composition  consists  of  two  parts  by  volume  of  hydrogen  to 
one  part  by  volume  of  oxygen. 


488  Voltaic  Electricity.  [466- 

466.  Electrolysis. — To  those  substances  which,  like  water,  are 
resolved  into  their  elements  by  the  voltaic  current,  the  term  elec- 
trolyte has  been  applied  by  Faraday,  to  whom  the  principal  dis- 
coveries in  this  subject,  and  the  nomenclature  are  due  ;  electrolysis 
is  the  decomposition  by  the  voltaic  battery. 

By  means  of  the  battery,  the  compound  nature  of  several  sub- 
stances which  had  previously  been  considered  as  elements  has  been 
determined.  By  means  of  a  battery  of  250  couples,  Davy,  shortly 
after  the  discovery  of  the  decomposition  of  water,  succeeded  in 
decomposing  the  alkalies  potass  and  soda,  and  proved  that  they 
were  the  oxides  of  the  hitherto  unknown  metals  potassium  and 
sodium.  The  decomposition  of  potass  may  be  demonstrated  with  the 

aid  of  the  battery  of 
four  to  six  elements  in 
the  following  manner  ; 
a  small  cavity  is  made 
in  a  piece  of  solid 
caustic  potass,  which 
is  moistened,  and  a 
drop  of  mercury  placed 
in  it  (fig.  386).  The 
potass  is  placed  on  a 

piece  of  platinum  con- 
Fig.  386. 

nected  with  the  posi- 
tive pole  of  the  battery.  The  mercury  is  then  touched  with  the 
negative  pole.  When  the  current  passes,  the  potass  is  decom- 
posed, oxygen  is  liberated  at  the  positive  pole,  while  the  potassium 
liberated  at  the  negative  pole  amalgamates  with  the  mercury.  On 
distilling  this  amalgam  out  of  contact  with  air,  the  mercury  passes 
off,  leaving  the  potassium. 

The  decomposition  of  binary  compounds,  that  is,  bodies  contain- 
ing two  elements,  is  quite  analogous  to  that  of  water  and  of  potass  ; 
one  of  the  elements  goes  to  the  positive,  and  the  other  to  the  nega- 
tive pole.  The  bodies  separated  at  the  positive  pole  are  called 
electronegative  elements,  because  at  the  moment  of  separation 
they  are  considered  to  be  charged  with  negative  electricity,  while 
those  separated  at  the  negative  pole  are  called  electropositive 
elements.  One  and  the  same  body  may  be  electronegative  or 
electropositive,  according  to  the  body  with  which  it  is  associated. 
For  instance,  sulphur  is  electronegative  towards  hydrogen,  but  is 
electropositive  towards  oxygen.  The  various  elements  may  be 


-467]  Electrotype.  489 

arranged  in  such  a  series  that  any  one  in  combination  is  electro- 
negative to  any  following,  but  electropositive  towards  all  preceding 
ones.  This  is  called  the  electrochemical  series,  and  begins  with 
oxygen  as  the  most  electronegative  element,  ending  with  potas- 
sium as  the  most  electropositive. 


APPLICATION   OF  THE  DECOMPOSITION   PRODUCED   BY  THE 
BATTERY. 

467.  Electrotype. — In  the  ordinary  methods  of  reproducing  in 
metal  statues,  basreliefs,  etc.,  moulds  of  dry  earth  are  prepared, 
which  are  faithful  hollow  copies  of  these_  .objects  ;  then  either 
melted  iron  or  bronze  are  run  into  these  ;  when  the  metal  is  solid, 
an  exact  copy  in  relief  is  obtained  of  the  object.  In  electrotypes, 
a  mould  of  the  object  to  be  produced  is  required,  but  the  repro- 
duction is  effected  without  either  fusion  or  fire.  The  current  of  a 
battery  quietly  deposits  a  layer  of  metal  of  any  desired  thickness 
on  a  faithful  impression  of  the  object.  This  is  the  meaning  of 
the  term  ofgalvanoplastics,  which  is  derived  from  the  word  galvanism, 
and  from  a  Greek  word  which  signifies  '  to  model.' 

The  practice  of  electrometallurgy  consists  of  two  distirlet  opera- 
tions ;  firstly,  the  preparation  of  the  mould  or  impression  of  the 
objects  to  be  reproduced  ;  and,  secondly,  the  deposit  of  the  metal  in 
this  mould.  The  first  process  is  the  most  delicate,  and  that  on 
which  mainly  depends  the  success  of  the  operation. 

Various  substances  are  used  for  taking  impressions,  wax,  stearine, 
fusible  metal,  gutta  percha,  etc.  Of  these  the  most  useful,  at  any 
rate  for  small  objects,  is  gutta  percha.  This  substance,  which  is 
hard  at  ordinary  temperatures,  softens  when  placed  in  warm  water. 
When  it  has  acquired  the  proper  degree  of  softness,  a  plate  of  this 
is  placed  on  the  object  to  be  copied  and  pressed  against  it.  When 
the  object  is  of  metal,  a  medal  for  instance,  the  gutta  percha  is 
easily  detached  as  soon  as  it  is  cold  ;  but  with  a  wood  engraving 
or  a  plaster  cast,  the  gutta  percha  adheres,  and  cannot  be  detached 
without  danger  of  tearing.  This  may  be  remedied  by  previously 
brushing  the  mould  over  with  black  lead  or  graphite,  as  it  ought  to 
be  called. 

Suppose  the  subject  to  be  reproduced  is  a  medal  (fig.  388),  when 
the  mould  is  obtained  we  have  this  metal  hollow  and  inverted.  It 
is  now  necessary  to  make  its  surface  a  conductor,  for  gutta  percha 
being  an  insulator  could  not  transmit  the  current  from  the  battery. 


490 


Voltaic  Electricity. 


[467- 


n 


This  is  effected  by  brushing  it  over  very  carefully  with  graphite 
(which  is  a  very  good  conductor)  in  all  those  places  where  the 
metal  is  to  be  deposited.  Three  copper  wires  are  then  fixed  to  it, 

one  of  which  is  merely  a  sup- 
port, while  the  two  others  con- 
duct the  current  to  the  metallic 
surface. 

The  mould  is  then  ready 
for  the  metal  to  be  deposited 
upon  it ;  copper  is  ordinarily 
used,  but  silver  and  gold  also 
deposit  well. 

In  order  to  take  a  copper 
cast,  a  bath  is  filled  with  satu- 


Fig.  387. 


Fig.  388. 


rated  solution  of  sulphate  of  copper  and  two  copper  rods,  B  and  A, 
stretched  across  (fig.  389)  :  one  connected  with  the  negative  and 
the  other  with  the  positive  pole  of  a  Grove's,  or  preferably,  from  its 
greater  constancy,  a  Daniell's  element.  From  the  rod  connected 
with  the  negative  pole,  B,  is  suspended  the  mould,  ;;z,  and  from  the 
other  A,  a  plate  of  copper,  C.  The  current  being  thus  closed,  the 
sulphate  of  copper  is  decomposed,  acid  is  liberated  at  the  positive 
pole,  while  copper  is  deposited  at  the  negative  pole,  on  the  mould 
suspended  from  the  rod,  B,  to  which  indeed  several  moulds  maybe 
attached. 

The   copper  plate  suspended  from  the  positive  pole  serves  a 
double  purpose  ;  it  not  only  closes  the  current,  but  it  keeps  the 


-468] 


Electrogilding. 


491 


solution  in  a  state  of  concentration,  for  the  acid  liberated  at  the 
positive  pole  dissolves  the  copper,  and  reproduces  a  quantity  of 
sulphate  of  copper  equal  to  that  which  has  been  decomposed.  The 


Fig.  389- 

bath  always  remains,  therefore,  at  the  same  degree  ot  concentration, 
that  is  to  say,  always  contains  the  same  amount  of  salt  in  solution, 
\\hich  is  a  condition  necessary  for  forming  a  uniform  deposit. 

468.  Electrogilding-. — The  old  method  of  gilding  was  by  means 
of  mercury.  It  was  effected  by  an  amalgam  of  gold  and  mercury, 
which  was  applied  on  the  metal  to  be  gilded.  The  objects  thus 
covered  were  heated  in  a  furnace,  the  mercury  volatilised,  and  the 
gold  remained  in  a  very  thin  layer  on  the  objects.  The  same 
process  was  used  for  silvering ;  but  they  were  expensive  and  un- 
healthy methods,  and  have  now  been  entirely  replaced  by  electro- 
gilding,  and  electrosilvering.  Brugnatelli,  a  pupil  of  Volta,  appears 
to  have  been  the  first,  in  1803,  to  observe  that  a  body  could  be  gilt 
by  means  of  the  battery  and  an  alkaline  solution  of  gold  ;  but  M. 
de  la  Rive  was  the  first  who  really  used  the  battery  in  gilding.  The 
methods  both  of  gilding  and  silvering  owe  their  present  high  state 
of  perfection  principally  to  the  improvements  of  Elkington,  Ruolz, 
and  others. 

The  difference  between  electrogilding  and  electrosilvering  and 
the  processes  described  in  the  previous  article  is  this,  that,  in  the 


492 


Voltaic  Electricity. 


[468- 


former,  the  metal  is  deposited  on  a  mould  in  order  to  reproduce 
the  objects  given  ;  while,  in  the  latter,  the  objects  are  permanently 
covered  with  a  thin  layer  of  gold  or  silver. 

The  pieces  to  be  coated  have  to  undergo  three  preparatory  pro- 
cesses. 

The  first  consists  in  heating  them  so  as  to  remove  the  tatty 
matter  which  has  adhered  to  thiem  in  previous  processes. 


Fig.  390. 

As  the  objects  to  be  gilt  are  usually  of  copper,  and  their  surface 
during  the  operation  of  heating  becomes  covered  with  a  layer  of 
suboxide  or  protoxide  of  copper,  this  is  removed  by  the  second 
operation.  For  this  purpose  the  objects,  while  still  hot,  are 
immersed  in  very  dilute  nitric  acid,  where  they  remain  until  the 
oxide  is  removed.  They  are  then  rubbed  with  a  hard  brush, 
washed  in  distilled  water,  and  dried  in  gently  heated  sawdust. 

To  remove  all  spots  they  must  undergo  the  third  process,  which 
consists  in  rapidly  immersing  them  in  ordinary  nitric  acid,  and 
then  in  a  mixture  of  nitric  acid,  bay  salt,  and  soot.  They  are  then 
well  washed  in  distilled  water,  and  dried  as  before  in  sawdust. 

When  thus  prepared  the  objects  are  attached  to  the  negative  pole 
of  a  battery  of  three  or  four  cells,  and  if  they  are  to  be  silvered 
they  must  be  immersed  in  a  bath  of  silver  kept  at  a  temperature  of 
sixty  to  eighty  degrees.  They  remain  in  the  bath  for  a  time  which 
depends  on  the  thickness  of  the  desired  deposit.  There  is  great 


-469]  Electromagnetism.  493 

difference  in  the  composition  of  the  bath.  That  most  in  use  con-. 
sists  of  two  parts  of  cyanide  of  silver  and  two  parts  of  cyanide  of 
potassium,  dissolved  in  250  parts  of  water.  In  order  to  keep  the 
bath  in  a  state  of  concentration,  a  piece  of  silver  is  suspended  from 
the  positive  electrode,  which  dissolves  in  proportion  as  the  silver 
dissolved  in  the  bath  is  deposited  on  the  objects  attached  to  the 
negative  pole. 

The  processes  of  electrogilding  are  quite  the  same  as  those  of 
electrosilvering,  with  the  exception  that  a  bath  of  gold  is  used  in- 
stead of  one  of  silver,  and  the  positive  plate  terminates  in  a  plate 
of  gold.  The  bath  used  is  a  solution  of  cyanide  of  gold  and  potas- 
sium. 

The  method  which  has  just  been  described  can  not  only  be  used 
for  gilding  copper,  but  also  for  silver,  bronze,  brass,  German  silver, 
etc.  But  other  metals,  such  as  iron,  steel,  zinc,  tin,  and  lead,  are 
very  difficult  to  gild  well.  To  obtain  a  good  coating  they  must  first 
be  covered  with  a  layer  of  copper  by  means  of  the  battery  and  a 
bath  of  sulphate  of  copper  ;  the  copper  with  which  they  are  coated 
is  then  gilded,  as  in  the  previous  case. 


CHAPTER    IX. 

ELECTROMAGNETISM. 

469.  Relation  between  magnetism  and  electricity. — Early 
in  the  history  of  the  two  sciences,  the  analogy  was  remarked  which 
existed  between  the  phenomena  of  electricity  and  magnetism.  It 
was  observed  that,  in  both  cases,  like  kinds  of  electricity  repelled 
each  other,  as  also  did  like  kinds  of  magnetism,  and  that  unlike 
kinds  attracted.  It  had  moreover  been  observed  that  lightning, 
in  striking  a  ship,  often  reversed  the  polarity  of  compass  needles, 
and  even  sometimes  robbed  them  of  all  magnetic  power.  But 
though  there  were  many  points  of  resemblance  between  electricity 
and  magnetism,  the  dissimilarities  were  numerous.  For  instance, 
magnetic  properties  cannot  be  transmitted  to  good  conductors,  as 
can  electrical  properties.  A  magnet  placed  in  contact  with  the 
•earth  does  not  lose  its  magnetism  as  does  an  electrified  body. 
Again,  electricity  can  be  produced  in  all  bodies,  while  magnetism 
is  only  manifested  by  a  very  small  number.  Among  these  resem- 


494 


Voltaic  Electricity. 


[469- 


blances  and  dissimilarities,  nothing  could  be  affirmed  respecting 
the  identity  of  the  causes  which  produce  electricity  and  magnetism, 
when,  towards  the  end  of  1819,  Oersted,  a  professor  of  physics  in 
Copenhagen,  made  a  memorable  discovery,  which  for  ever  inti- 
mately connected  these  two  physical  agents.  Thus  arose  a  new 
branch  of  science  called  electromagnetism,  to  express  that  the 
phenomena  are  at  once  magnetic  and  electrical. 

470.  Action  of  current  upon  magnets. — The  fact  which 
Oersted  discovered  was  the  directive  action  of  currents  upon 
magnets.  He  found  that  electrical  currents  have  a  directive  action 
upon  the  magnetic  needle,  and  always  tend  to  set  it  at  right  angles 
to  their  own  direction. 


Fig.  39i- 

To  verify  this  action  of  currents  upon  magnets,  the  experiment  is 
arranged  as  shown  in  fig.  391.  A  magnetic  needle,  movable  upon  a 
pivot,  being  at  rest  in  the  direction  of  the  magnetic  meridian,  a 
wire  traversed  by  a  current  is  brought  near  it,  care  being  taken  to 
bring  it  lengthways.  The  needle  is  then  seen  to  deviate  from  its 
position  of  rest,  to  oscillate,  and  ultimately  come  to  rest  in  a 
position  which  is  nearly  at  right  angles  to  that  of  the  current ;  and 
the  more  nearly,  the  more  powerful  the  current. 

In  this  experiment  the  direction  in  which  the  north  pole  is  de- 
flected varies  with  the  direction  of  the  current ;  if  it  goes  from 
south  to  north  above  the  needle,  the  north  pole  is  deflected  to  the 


-472] 


Action  of  Magnets  on  Currents. 


495 


west ;  if,  on  the  contrary,  it  goes  from  north  to  south  but  still  above 
the  needle  the  north  pole  is  deflected  to  the  east.  When  the  cur- 
rent passes  below  the  needle,  the  same  phenomena  are  reproduced 
in  exactly  the  reverse  order.  All  these  different  cases  have  been 
reduced  to  a  single  one  by  Ampere. 

471.  Ampere's  rule. — Ampere  has  given  the  following  memoria 
technica,  by  which  all  the  various  directions  of  the  needle  under  the 
influence  of  a  current  may  be  remembered.  If  we  imagine  an 
observer  placed  in  the  connecting  wire  in  such  a  manner  that  the 
current  entering  by  his  feet  issues  by  his  head,  and  that  his  face  is 
always  turned  towards  the  needle,  we  shall  see  that  in  the  above 
four  positions,  the  north  pole  is  always  deflected  towards  the  left  of 
the  observer.  By  thus  personifying  the  current,  the  different  cases 
may  be  comprised  in  this  general  principle  :  In  the  directive  action 
of  currents  on  magnets,  the  north  pole  is  always  deflected  towards 
the  left  of  the  current. 

47?.  Action  of  magnets  and  of  the  earth  on  currents. — Just 
as  currents  act  on  magnets,  so  also  magnets  act  upon  currents.  To 


Fig.  392. 

prove  this,  a  circle  of  copper  wire  provided  at  the  ends  with  steel 
points  dip  in  two  mercury  cups  (fig.  392).  These  mercury  cups 
are  at  the  ends  of  two  metal  rods  attached  to  two  vertical  columns, 
with  which  can  be  connected  the  poles  of  Bunsen's  element.  By 
this  arrangement,  which  is  known  as  Ampere's  stand,  we  have  a 
movable  circuit  continually  traversed  by  a  current.  When  this 
circuit  is  at  rest,  if  a  powerful  magnet  be  placed  beneath  the  circuit, 


496 


Voltaic  Electricity. 


[472 


but  in  its  plane,  the  circuit  will  be  seen  to  turn  and  set  transversely 
to  the  bar,  which  is  the  converse  of  Oersted's  experiment. 

The  terrestrial  globe,  which  acts  like  a  magnet  on  magnetic 
needles,  acts  in  the  same  manner  on  the  movable  circuits,  that  is, 
it  directs  them  perpendicularly  to  the  magnetic  meridian.  This 
action  may  be  demonstrated  by  the  above  apparatus.  With  this 
view,  before  the  current  traverses  the  circuit,  it  is  placed  in  the 
magnetic  meridian,  and  then  the  two  poles  of  the  battery  are  con- 
nected with  the  two  columns  :  the  circuit  is  soon  observed  to  set 
transversely  to  its  first  position,  and  so  that,  in  the  lower  part  of  the 
circuit,  the  direction  of  the  current  is  from  east  to  west. 

473.  Galvanometer,  or  multiplier. — The  name  galvanometer 
multiplier,  or  rheometer  is  given  to  a  very  delicate  apparatus,  by 
which  the  ex;stence,  direction,  and  intensity  of  currents  may  be  de- 
termined. It  was  invented  by  Schweigger  in  Germany  a  short 
time  after  Oersted's  discovery. 

In  order  to  understand  its  principle,  let  us  suppose  a  magnetic 
needle,  ab,  suspended  by  a  filament  of  silk  (fig.  393),  and  surrounded 
in  the  plane  of  the  magnetic  meridian  by  a  copper  wire  forming  a 


Fig.  393- 


complete  circuit  round  the  needle  in  the  direction  of  its  length. 
When  this  wire  is  traversed  by  a  current,  it  follows,  from  what  has 
been  said  in  the  previous  paragraph,  that  in  every  part  of  the  circuit 
an  observer  lying  in  the  wire  in  the  direction  of  the  arrows,  and 
looking  at  the  needle,  ab,  would  have  his  left  always  turned  towards 
the  same  point  of  the  horizon,  and  consequently,  that  the  action  of 
the  current  in  every  part  would  tend  to  turn  the  north  pole  in  the. 
same  direction  :  that  is  to  say,  that  the  actions  of  the  four  branches 
of  the  circuit  .concur  to  give  the  north  pole  the  same  direction.  By 
coiling  the  copper  wire  in  the  direction  of  the  needle,  as  represented 


-473] 


Galvanometer. 


497 


in  the  figure,  the  action  of  the  current  has  been  multiplied.  If 
instead  of  a  single  one,  there  are  several  circuits,  provided  they  are 
insulated,  the  action  becomes  still  more  multiplied,  and  the  deflec- 
tion of  the  needle  increases  ;  or,  what  is  the  same  thing,  a  much 
feebler  current  will  produce  deflection. 

As  the  directive  action  of  the  earth  continually  tends  to  keep  the 
needle  in  the  magnetic  meridian,  and  thus  opposes  the  action  of  the 
current,  the  effect  of  the  latter  is  increased  by  using  an  astatic 
system  of  two  needles  as  shown  in  fig.  394.  The  action  of  the 
earth  on  the  needle  is  then  very  feeble,  and,  further,  the  actions  of 
the  current  on  the  two  needles  become  accumulated.  In  fact,  the 
action  of  the  circuit,  from  the  direction  of  the  current  indicated  by 
the  arrows,  tends  to  deflect  the  north  pole  of  the  lower  needle  ab 
towards  the  west.  The  upper  needle,  a'  bf,  is  subjected  to  the  action 


Fig.  395- 

ot  two  contrary  currents,  no  and  qp,  but  as  the  first  is  nearer,  its 
action  preponderates.  Now  this  current,  passing  below  the  needle, 
evidently  tends  to  turn  the  pole,  of,  towards  the  east,  and  conse- 
quently, the  pole,  b',  towards  the  west  ;  that  is  to  say,  in  the  same 
direction  as  the  pole,  a,  of  the  other  needle. 

K  K 


498  Electromagnetism.  [473 

From  these  principles  it  will  be  easy  to  understand  the  theory  of 
the  multiplier.  The  apparatus,  represented  in  fig.  395,  consists  of  a 
thick  brass  plate  resting  on  levelling  screws  ;  on  this  is  a  copper 
frame  on  which  is  coiled  a  great  number  of  turns  of  wire  covered 
with  silk.  The  two  ends  terminate  in  binding  screws,  n  and  m. 
Above  the  frame  is  a  graduated  circle,  with  a  central  slit  parallel 
to  the  direction  in  which  the^vire  is  coiled.  By  means  of  a  very 
fine  filament  of  silk,  an  astatic  system  is  suspended  ;  it  consists  of 
two  needles,  ab  and  a!  b' ',  one  above  the  scale,  and  the  other  within 
the  circuit  itself. 

In  using  the  instrument  it  is  so  adjusted  that  the  needles,  and 
also  the  slit,  are  in  the  magnetic  meridian. 

474.  Uses    of  the    galvanometer. — To    show,    by  means    of 
the  multiplier,  the  electricity  developed  in  chemical  actions,  for 
instance  in  the  action  of  acids   on  metals,   two   platinum   wires 
may  be  attached  to  the  binding  screws,  m  and  n.     Then  one  of 
them  is    plunged   in  very  dilute   sulphuric   acid,   and  the   other 
placed  in  contact  with  a  piece  of  zinc  held  in  the  hand,  which  is 
dipped  in  the  liquid.     An  immediate  deflection  is  observed,  which 
indicates  the  existence  of  a  current  :  and  from  the  direction  which 
the  north  pole  of  each  needle  assumes,  it  is  seen  that  the  direction 
of  the  current  is  that  indicated  by  the  arrows.     From  which  we  may 
conclude,  in  accordance  with  the  explanation  given  as  to  the  origin 
of  electricity  in  the  simple  voltaic  circuit,  that  the  acid  is  positively 
electrified  and  the  zinc  negatively. 

The  length  and  diameter  of  the  wire  vary  with  the  purpose  for 
which  the  galvanometer  is  intended.  For  one  which  is  to  be  used 
in  observing  the  currents  due  to  chemical  actions,  a  wire  about  \ 
millimetre  in  diameter,  and  making  about  800  turns,  is  well 
adapted.  Those  for  thermo-electric  currents,  which  have  low  inten- 
sity, require  a  thicker  and  shorter  wire,  for  example,  thirty  turns  of 
a  wire  f  millimetre  in  diameter.  For  very  delicate  experiments,  as 
in  physiological  investigations,  galvanometers  with  as  many  as  30,000 
turns  have  been  used. 

475.  Magnetisation  by  electrical    currents. — From  the   in- 
fluence which  currents  exert  upon  magnets,  turning  the  north  pole 
co  the  left  and  the  south  pole  to  the  right,  it  is  natural  to  think,  that 
by  acting  upon  magnetic  substances  in  the  natural  state  the  currents 
would   tend   to   separate  the  two  magnetisms.     In  fact,  when   a 
wire  traversed  by  a  current  is  immersed  in  iron  filings,  they  adhere 
to  it  in  large  quantities,  but  become   detached   as   soon   as  the 


-476]         Magnetisation  by  Electrical  Currents.  ,499 

current  ceases,  while  there  is  no  action  on  any  other  non-magnetic 
metal. 

The  action  of  electrical  currents  on  magnetic  substances  is  well 
seen  in  an  experiment  due  to  Ampere,  which  consists  in  coiling  an  in- 
sulated copper  wire  round  an  unmagnetised  steel  bar.  If  a  current 


Fig.  396. 

be  passed  through  the  wire,  even  for  a  short  time,  the  bar  becomes 
strongly  magnetised.  The  same  effect  is  produced  with  a  bar  of 
soft  iron,  but  in  this  case  the  magnetisation  is  temporary  ;  when 
the  current  ceases,  the  iron,  which  is  destitute  of  coercive  force,  re- 
verts instantaneously  to  the  natural  state  ;  and,  if  in  this  experi- 
ment, we  imagine  an  observer  floating  in  the  direction  of  the 
current,  the  north  pole  is  always  on  his  left  hand. 

If  the  charge  of  a  Leyden  jar  be  transmitted  through  the  wire 
by  connecting  one  end  with  the  outer  coating,  and  the  other  with 
the  inner  coating,  the  bar  is  also  magnetised.  Hence  both  voltaic 
and  frictional  electricity  can  be  used  for  magnetising. 


CHAPTER   X. 

ELECTRODYNAMICS. 

476.  Reciprocal  action  of  currents  on  currents. — Ampere 
did  not  restrict  himself  to  trying  the  action  of  magnets  and  of  the 
earth  upon  movable  currents  ;  he  went  further,  and  was  led  to  the 
important  discovery,  that  electrical  currents  act  on  each  other  as  do 
magnets  ;  and  he  thus  created  an  entirely  new  branch  of  physics, 
to  which  the  name  electrodynamics  has  been  given.  The  actions 
which  currents  exert  on  each  other  are  different  according  as  they 
are  parallel  or  angular. 

I.  Two  currents  which  are  parallel,  but  in  contrary  directions^ 
repel  each  other. 

KK  2 


Electrodynamics. 


[476- 


II.  Two  currents ;  parallel  and  in  the  same  direction,  attract 
e(ich  other. 

To  verify  these  laws  use  is  made  of  the  apparatus  represented 
in  fig.  397.  On  a  wooden  support  are  fixed  two  brass  columns,  A  and 

B,  joined  at  the  top  by 
a  wooden  cross-piece. 
In  the  centre  of  this 
is  a  brass  binding 
screw,  a,  and  below 
this  a  mercury  cup,  o. 
In  this  is  placed  an 
iron  pivot  which  joins 
the  end  of  a  copper 
wire.  This  wire  is 
coiled  in  the  manner 
represented  in  the 
figure,  terminating  in 
a  mercury  cup,  c,  on  the 
base  of  the  apparatus. 
It  thus  forms  a  circuit 
Fig.  397-  movable  about  the 

pivot. 

This  being  premised,  the  circuit  is  arranged  in  the  plane  of  the 
two  columns,  as  shown  in  fig.  397,  and  the  current  from  a  Bunsen's 
battery  is  passed  through  it  to  the  foot  of  the  column,  A  ;  it  passes 
thence  by  a  copper  wire  to  the  binding  screw,  a  ;  thence  into  the  cup, 
ao,  traverses  the  entire  movable  circuit  in  the  direction  of  the 
arrows,  reaches  the  cup,  C,  whence,  by  a  copper  strip,  it  reaches  the 
foot  of  the  column,  B,  rises  in  this,  and  ultimately  returns  to  the 
battery.  When  the  current  passes,  the  circuit  moves  away  from  the 
columns,  and,  after  a  few  oscillations,  comes  to  rest  crosswise  to  its 
original  position  ;  thus  showing  that  the  ascending  current  in  the 
columns  and  the  descending  current  in  the  circuit  repel  each  other, 
thereby  proving  the  first  law. 

The  second  law  may  be  established  by  means  of  the  same  ap- 
paratus, replacing  the  movable  circuit  depicted  in  fig.  397  by 
another  so  arranged  that  the  current  is  ascending  in  both  the 
columns  and  in  the  two  branches  of  the  circuit.  When  the  mov- 
able circuit  is  displaced,  and  the  current  is  passed,  the  latter  returns 
briskly  towards  the  columns. 

Law  of  angidar  currents.     In  the  case  of  two  angular  currents, 


-479]  Solenoids.  501 

one  fixed  and  the  other  movable,  Ampere  found  that  there  was 
attraction  when  both  the  currents  moved  towards,  or  both  away 
from,  the  apex  of  the  angle  ;  and  that  repulsion  took  place  when, 
one  current  moving  towards  the  apex,  the  other  moved  away 
from  it. 

SOLENOIDS. 

477.  structure  of  a  solenoid. — A  solenoid  is  a  system  of  equal 
and  parallel  circular  currents  formed  of  the  same  pieces  of  covered 
copper  wire,  and  coiled  in  the 

form  of  a  helix  or  spiral,  as  re- 
presented in  fig.  398.  A  sole- 
noid, however,  is  only  complete 
when  part  of  the  wire,  BC,  passes 
in  the  direction  of  the  axis  in 
the  interior  of  the  helix.  With 
this  arrangement,  when  the 
circuit  is  suspended  in  the  mer-  Fig  398> 

cury  cups,  abt  of  the  apparatus 

(ng.  397),  and  a  current  is  passed  through,  it  is  directed  by  the 
earth  exactly  as  if  it  were  a  magnetic  needle.  If  the  solenoid 
be  removed  it  will,  after  a  few  oscillations,  return,  so  that  its 
axis  is  in  the  magnetic  meridian.  Further,  it  will  be  found  that, 
in  the  lower  half  of  the  coils  of  which  the  solenoid  consists,  the 
direction  of  the  current  is  from  east  to  west ;  in  other  words,  the 
current  is  descending  on  that  side  of  the  coil  turned  towards  the 
east,  and  ascending  on  the  west.  In  this  experiment  the  solenoid 
is  directed  like  a  magnetic  needle,  and  the  north  pole,  as  in  mag- 
nets, is  that  end  which  points  towards  the  north,  and  the  south 
pole  that  which  points  towards  the  south. 

478.  Mutual  actions  of  magnets  and  solenoids. — Exactly  the 
same  phenomena  of  attraction  and  repulsion  exist  between  solenoids 
and  magnets  as  between  magnets.     For  if  to  a  movable  solenoid 
traversed  by  a  current,  one  of  the  poles  of  a  magnet  be  presented, 
attraction  or  repulsion  will  take  place,  according  as  the  poles  of  the 
magnet  and  of  the  solenoid  are  of  contrary,  or  of  the  same  name. 
The  same  phenomenon  takes  place  when  a  solenoid,  traversed  by 
a  current  and  held  in  the  hand,  is  presented  to  a  movable  mag- 
netic needle.     Hence  the  law  of  attractions  and  repulsions  applies 
exactly  to  the  case  of  the  mutual  action  of  solenoids  and  of 
magnets. 

479.  Mutual  actions  of  solenoids.— When  two  solenoids  tra- 


502 


Electrodynamics. 


[479- 


versed  by  a  powerful  current  are  allowed  to  act  on  each  other,  one 
of  them  being  held  in  the  hand,  and  the  other  being  movable 
about  a  vertical  axis,  as  shown  in  fig.  399,  attraction  and  repulsion 


Fig.  399- 

will  take  place,  just  as  in  the  case  of  two  magnets.  These  pheno- 
mena are  readily  explained  by  reference  to  what  has  been  said 
about  the  mutual  actions  of  the  currents,  bearing  in  mind  the  direc- 
tion of  the  currents  in  the  ends  presented  to  each  other. 

480.  Ampere  s  theory  of  magnetism. — Ampere  has  propounded 
a  most  ingenious  theory,  based  on  the  analogy  which  exists  be- 
tween solenoids  and  magnets,  by  which  all  magnetic  phenomena 
may  be  referred  to  electrodynamical  principles. 

Instead  of  attributing  magnetic  phenomena  to  the  existence  of 
two  fluids,  Ampere  assumes  that  each  individual  molecule  of  a 
magnetic  substance  is  traversed  by  a  closed  electric  current. 
When  the  magnetic  substance  is  not  magnetised,  these  molecular 
currents,  under  the  influence  of  their  mutual  attractions,  occupy 
such  positions  that  their  total  action  on  any  external  substance  is 
null.  Magnetisation  consists  in  giving  to  these  molecular  currents 
a  parallel  direction,  and  the  stronger  the  magnetising  force  the 
more  perfect  the  parallelism.  The  limit  of  magnetisation  is  at- 
tained when  the  currents  are  completely  parallel. 

The  resultant  of  the  actions  of  all  the  molecular  currents  is 
equivalent  to  that  of  a  single  current  which  traverses  the  outside  of 
a  magnet.  For  by  inspection  of  fig.  400,  in  which  the  molecular 
currents  are  represented  by  a  series  of  small  internal  circles  in 
the  two  ends  of  a  cylindrical  bar,  it  will  be  seen  that  the  adjacent 


-481] 


Ampere's  Theory  of  Magnetism. 


503 


parts  of  the  currents  oppose  one  another,  and  cannot  exercise  any 
external  electrodynamic  action,  which  is  not  the  case  with  those  on 
the  surface. 

The  direction  of 
these  currents  in  mag- 
nets can  be  ascer- 
tained by  considering 
the  suspended  sole- 
noid (fig.  398).  If  we 
suppose  it  traversed  by 
a  current,  and  in  equi- 
librium in  the  magne- 
tic meridian,  it  will  set 
in  such  a  position  that 
in  the  lower  half  of 
each  coil  the  current  flows  from  east  to  west.  We  may  then  esta- 
blish the  following  rule.  At  the  north  pole  (English]  of  a  magnet 
the  direction  of  the  Amptrian  currents  is  opposite  that  of  the  hands 
of  a  watch)  and  at  the  south  pole  the  direction  is  the  same  as  that  oj 
the  hands. 

481.  Terrestrial  current. — In  order  to  explain  on  this  supposi- 
tion tenestrial  magnetic  effects,  the  existence  of  electrical  currents 
is  assumed  which  continually  circulate  round  our  globe  from  east 
to  west,  perpendicular  to  the  magnetic  meridian.  . 

The  resultant  of  their  action  is  a  single  current  traversing  the 
magnetic  equator  from  east  to  west.  These  currents  are  supposed 
to  be  thermo-electric  currents  due  to  the  variations  of  temperature 
caused  by  the  successive  influence  of  the  sun  on  the  different  parts 
of  the  globe  from  east  to  west. 

These  currents  direct  magnetic  needles  ;  for  a  suspended  mag- 
netic needle  comes  to  rest  when  the  molecular  currents  on  its 
under  surface  are  parallel,  and  in  the  same  direction  as  the  earth 
currents.  As  the  molecular  currents  are  at  right  angles  to  the 
direction  of  its  length,  the  needle  places  its  greatest  length  at  right 
angles  to  east  and  west,  or  north  and  south.  Natural  magnetisa- 
tion is  probably  imparted  in  the  same  way  to  iron  minerals. 


504 


Electromagnets. 


[482- 


CHAPTER  XI. 

ELECTROMAGNETS.      TELEGRAPHS  AND   ELECTROMAGNETIC 
MOTORS. 

482.  Electromagnets — Electromagnets  are   bars  of  soft    iron 
which,  under  the  influence  of  a  voltaic  current,  become  magnets ; 

but  this  magnetism  is  only  tempo- 
rary, for  the  coercive  force  of  per- 
fectly soft  iron  is  null,  and  the 
magnetism  ceases  as  soon  as  the 
current  ceases  to  pass  through  the 
wire.  If,  however,  the  iron  is  not 
quite  pure,  it  retains  more  or  less 
traces  of  magnetism.  The  electro- 
magnets have  the  horse-shoe  form, 
as  shown  in  fig.  401,  and  a  copper 
wire,  covered  with  silk  or  cotton,  is 
rolled  several  times  round  them  on 
the  two  branches,  so  as  to  form 
two  bobbins,  A  and  B.  In  order 
that  the  two  ends  of  the  horse-shoe  may  be  of  opposite  polarity  the 
winding  on  the  two  limbs,  A  and  B,  must  be  such  that,  if  the  horse- 
shoe were  straightened  out,  it  would  be  in  the  same  direction. 

Electromagnets,  instead  of  being  made  in  one  piece,  are  fre- 
quently constructed  of  two  cylinders,  firmly  screwed  to  a  stout 
piece  of  the  same  metal.  Such  are  the  electromagnets  in  Morse's 
telegraph  (487),  the  electromagnetic  machine  (502).  The  helices  on 
them  must  He  such  that  the  current  shall  flow  in  the  same  direction 
as  the  hand  of  a  watch  as  seen  from  the  south  pole,  and  against  the 
hands  of  a  watch  as  seen  from  the  north  pole. 

The  force  of  such  magnets  depends  on  their  dimensions,  on  the 
number  of  turns  of  wire,  and  on  the  strength  of  the  current.  An 
electromagnet  need  not  be  very  powerful  to  support  one  person 
(fig.  402).  Electromagnets  have  extended  applications,  in  tele- 
graphs, in  clocks,  and  in  electromagnetic  engines. 


Fig.  401. 


-483] 


Electric  Telegraph. 


505 


ELECTRIC  TELEGRAPH. 


483.  Electric  telegraphs. — These  are  apparatus  by  which 
signals  can  be  transmitted  to  considerable  distances,  and  with 
enormous  velocity,  by  means  of  voltaic  currents  propagated  in 


Fig.  402. 

metal  wires.  Towards  the  end  of  the  last  century,  and  at  the 
beginning  of  the  present,  many  philosophers  proposed  to  corre- 
spond at  a  distance  by  means  of  the  effects  produced  by  electrical 
machines  when  propagated  in  insulated  conducting  wires.  In 
181 1,  Scemmering  invented  a  telegraph  in  which  he  used  the  de- 
composition of  water  for  giving  signals.  In  1820,  at  a  time  when 


506  Electric  Telegraph.  [483- 

« 

the  electromagnet  was  unknown,  Ampere  proposed  to  correspond 
by  means  of  magnetic  needles,  above  which  a  current  was  sent,  as 
many  wires  and  needles  being  used  as  letters  were  required.  In 
1 834,  Gauss  and  Weber  constructed  an  electromagnetic  telegraph, 
in  which  a  voltaic  current  transmitted  by  a  wire  acted  on  a  magnet- 
ised bar  ;  the  oscillations  of  which  under  its  influence  were  ob- 
served by  a  telescope.  They  succeeded  in  thus  sending  signals 
from  the  Observatory  to  the  Physical  Cabinet  in  Gottingen,  a  dis- 
tance of  a  mile  and  a  quarter,  and  to  them  belongs  the  honour  of 
having  first  demonstrated  experimentally  the  possibility  of  electrical 
communication  at  a  considerable  distance.  In  1837,  Steinheil  in 
Munich,  and  Wheatstone  in  London,  constructed  telegraphs  in 
which  several  wires  each  acted  on  a  single  needle  :  the  current  in 
the  first  case  being  produced  by  an  electromagnetic  machine,  and 
in  the  second  by  a  constant  battery. 

Every  electric  telegraph  consists  essentially  of  three  parts  :  I,  a 
circuit,  consisting  of  a  metallic  connection  between  two  places,  and 
an  electromotor,  for  producing  the  current ;  2,  a  communicator,  for 
sending  the  signals  from  one  station  ;  and,  3,  an  indicator,  for 
receiving  them  at  the  other  station.  The  manner  in  which  these 
objects,  especially  the  last  two,  are  effected  can  be  greatly  varied  ; 
the  three  principal  systems  are  the  needle  telegraph,  the  dial  tele- 
graph, and  the  printing  telegraph. 

The  needle  telegraph  is  essentially  a  vertical  galvanometer  ;  that 
is  to  say,  a  magnetic  needle  suspended  vertically  in  a  coil  of  insu- 
lated wire.  To  the  needle  is  attached  an  index,  which  is  seen  on 
the  front  of  the  apparatus.  The  signs  are  made  by  transmitting 


Fig.  403. 

the  current  in  different  directions  through  the  multiplier,  by  which 
the  needle  is  deflected  either  to  the  right  or  left,  according  to  the 


--484]  Principle  of  Morses  Telegraph.  507 

will  of  the  operator.  The  instrument  by  which  this  is  effected  is 
called  a  key,  or  commutator. 

In  the  dial  telegraph  an  electromagnet  causes  an  idex  to  move 
over  a  dial  provided  with  the  twenty-six  letters  of  the  alphabet  ; 
that  letter  in  front  of  which  the  needle  stops,  being  the  letter  sent. 
By  this  kind  of  telegraph  messages  are  not  sent  with  great  rapid- 
ity ;  yet,  as  the  manipulation  is  very  simple,  it  is  frequently  used 
on  railways  and  in  private  offices. 

484.  Principle  of  Morse's  telegraph. — This  telegraph  is  based 
on  the  temporary  magnetisation  of  an  electromagnet  by  the  inter- 
mittent passage  of  currents.  Thus  let  E  (fig.  403)  be  a  fixed 
electromagnet,  the  insulated  wires  of  which  are  attached  to  the  two 
binding  screws,  a  and  b.  Above  this  magnet  is  a  lever,  mn,  movable 
about  an  axis,  i,  and  ending  in  an  armature  of  soft  iron,  m,  so  that, 
whenever  the  magnet  is  traversed  by  a  current,  the  armature  is 
attracted,  and  the  part  of  the  lever  on  the  right  of  the  fulcrum  is 
lowered  ;  then,  when  the  current  no  longer  passes,  a  spring,  R, 
raises  the  lever  to  an  extent  regulated  by  a  screw,  O. 

Suppose,  for  example,  the  electromagnet  is  at  Bristol,  and  that 
there  is  a  battery,  P,  at  London,  and  two  metal  wires,  A  and  B,  by 
one  of  which  the  binding  screw,  $,  is  permanently  connected  with 
the  negative  pole  of  the  battery,  while  the  experimenter  holds  the 
other  wire  in  his  hand.  So  long  as  the  experimenter  does  not 
place  the  wire  which  he  holds  in  his  hand  in  contact  with  the  posi- 
tive pole,  the  current  does  not  pass  ;  and,  as  the  electromagnet  does 
.not  act,  the  arm  of  the  lever  is  raised  (fig.  403).  But  the  moment 


Fig.  404. 

contact  is  made,  the  current  is  closed,  the  electromagnet  attracts, 
and  the  lever  is  lowered  (fig.   404) ;    but   it   resumes  its  original 


Electric  Telegraph. 


[484- 


position  as  soon  as  contact  is  broken,  and  so  on  at  the  will  of  the 
operator.  Thus  one  person  at  London  can  cause  the  lever,  mn,  to 
oscillate  at  Bristol  as  often  and  as  rapidly  as  possible  as  he  desires. 
This  is,  in  its  simplest  form,  the  principle  of  the  elementary  mech- 
anism of  electrical  telegraphs  based  on  electromagnetism.  It  only 
remains  to  give  to  these  oscillations  a  definite  meaning. 

485.  line  wire. — Of  the  various  essentials  for  a  telegraphic  com- 
munication, the  batteries  or  sources  of  power  have  been  already 
described,  and  we  shall  therefore  pass  to  the  explanation  of  the 
circuit,  or  line  wire. 

Line  wires  are  either 
aerial,  subterranean,  or  sub- 
marine. 

The  aerial  wire  consists  of 
a  stout  galvanised  iron  wire 
connecting  two  stations.  At 
certain  intervals  are  wooden 
posts,  to  which  are  attached 
insulating  supports  of  por- 
celain, which  sustain  the 
wire  (fig.  405).  Subterranean 
wires  are  used  for  cases  in 
which  an  aerial  wire  would 

not  be  sufficiently  protected  against  accident,  as  in  towns.  They 
consist  usually  of  copper  wires  covered  with  gutta  percha  ;  this 
insulates  them  from  the  earth  in  which  they  are  placed. 

Submarine  wires  or  cables  are  such  as  are  employed  in  deep  seas 
where  great  strength  is  required.  The  ordinary  form  is  represented  in 
figs.  406  and  407.  The  core  consists  of  seven  fine  wires  of  very  pure 


Fig.  406. 


Fig.  407. 


copper,  which  are  twisted  together  and  surrounded  by  an  insulating 
covering.  This  is  surrounded  by  an  insulating  coating  of  four 
concentric  layers  of  gutta  percha  alternating  with  the  same  number 


-486]  The  Earth  as  a  Conductor.  509 

of  layers  of  a  material  known  as  Chattertorts  compound,  which  is 
essentially  a  mixture  of  resin,  pitch,  and  gutta  percha  applied  hot. 
Round  this  is  a  layer  of  tarred  hemp,  and  this  again  is  surrounded 
by  a  protective  coating  of  steel  wire  coated  with  tarred  hemp,  which 
preserves  it  from  the  corrosive  action  of  the  sea. 

Fig.  406  gives  a  longitudinal  view  of  a  submarine  cable,  and 
fig.  407  a  cross  section.  The  diameter  of  the  cable  is  about  an 
inch,  and  it  weighs  about  a  ton  to  the  mile. 

486.  The  ear tii  as  a  conductor. — In  figs.  403  and  404  we 
have  not  merely  a  wire  connecting  the  positive  pole  of  the  battery 
with  the  electromagnet,  but  there  is  a  second  one  which  acts  on  a 
return  wire.  In  1837  Steinheil  made  the  very  important  discovery 
that  the  earth  might  be  utilised  for  the  return  conductor.  This  has 
the  twofold  advantage  of  doing  away  with  the  expense  of  a  second 
wire,  and  also  of  lessening  the  resistance. 

With  this  view,  at  the  sending  station,  a  long  copper  wire  is 
attached  to  the  negative  pole,  which  is  fixed  at  the  other  end  to  a 
copper  plate,  Q.  This  plate  is  placed  in  water  if  possible  (fig.  408),  or 


Fig.  408. 

at  all  events  is  sunk  some  depth  in  earth.  In  like  manner,  at  the 
receiving  station,  a  similar  wire  and  plate  s  are  connected  with  the 
binding  screw,  b.  Thus  while  the  negative  electricity  passes  into 
the  ground  by  the  plate,  Q,  the  positive  electricity  which  reaches 
the  electromagnet  and  the  binding  screw,  enters  the  ground  by  the 
plate,  S.  Hence  there  is  in  the  wire,  A,  and  in  the  electromagnet, 
the  same  circulation,  and  therefore  the  same  effects  as  when  the 
binding  screw,  b,  communicates  directly  with  the  negative  pole  of 
the  battery  by  means  of  a  metal  wire. 


5io 


Electric  Telegraph. 


[487- 


487.  Morse's  telegraph. — Fig.  409  represents  a  station  at  which 
a  despatch  is  being  sent  by  the  help  of  this  apparatus,  and  fig.  410 
represents  the  receiving  station.  At  each  station  the  apparatus  is 
the  same  ;  it  is  double,  and  consists  of  two  distinct  parts,  the  key,  by 
which  the  signals  are  sent,  and  the  receiving  instrument  which 
registers  them.  These  two  parts  are  represented  on  a  larger  scale 
in  figs.  411  and  412. 

To  understand  how  they  work  let  us  commence  with  fig.  409. 
Below  the  table  is  a  box  containing  the  battery,  which  furnishes  the 


Fig.  409. 

current.  This  passes  by  the  wire,  P,  into  the  key,  which  will  be  after- 
wards described  (fig.  411).  Thence  it  passes  into  a  small  galvano- 
meter, £,  which  indicates  by  the  deflection  of  its  needle  whether  the 


-488] 


Morses  Telegraph. 


current  is  passing  or  not.  The  current  ultimately  attains  the 
piece,  M,  which  acts  as  a  lightning  conductor,  as  we  shall  afterwards 
see,  and  thence  it  goes  to  the  wire,  L,  which  is  the  line  wire. 

This  wire  is  again  seen  at  the  top  of  fig.  410,  whence  the  arriving 
current  again  passes  into  the  lightning  conductor,  then  into  a  gal- 


Fig.  410. 


vanometer,  and  next  a  key,  whence  it  passes  into  the  electromag- 
net, which  makes  part  of  the  receiver.  It  then  enters  the  wire,  T, 
which  leads  it  to  earth. 

488.  Morse's  key  and  receiving:  instrument. — The  general 
arrangement  of  the  apparatus  being  understood,  the  following  are 
the  details  of  its  action.  The  key  consists  of  a  small  mahogany 
base,  which  acts  as  support  for  a  metallic  lever,  hk  (fig.  411),  mov- 


512 


Electric  Telegraph. 


[488- 


able  in  its  middle  on  a  horizontal  axis.     The  extremity,  B,  of  this 
lever  is  always  pressed  upwards  by  a  spring,  r,  beneath  ;  at  the  other 

end  a  screw  passes 
through  it,  which  rests 
on  a  small  metal  sup- 
port, in  contact  with 
the  wire,  A.  Fig.  41 1 
represents  the  key 
at  the  moment  it  re- 
Fig.  411.  ceives  the  dispatch,  as 
at  work  for  instance  in  fig.  410.  The  current  enters  then  by  the 
wire,  L,  which  is  the  line  wire,  rises  into  the  lever,  kh,  and  de- 
scends by  the  screw  pin,  a,  into  the  wire,  A,  which  leads  to  the  indi- 
cator. If,  on  the  other  hand,  the  key  is  to  be  used  for  sending  a 
message,  as  represented  in  fig.  409,  it  will  be  seen  that  the  lever, 
kh,  does  not  touch  the  metal  pin  in  which  the  wire,  P,  terminates. 
But  if  the  lever,  h,  is  lowered  by  pressing  the  end,  B,  contact  is  set 
up,  and  the  current,  P,  at  once  passes  into  the  lever,  hk,  and  thence 
into  the  wire,  L,  which  leads  it  to  the  station  signalled  to  ;  for  the 
same  wire  is  used  to  send  and  to  receive  the  message. 

The  indicator  consists  of  an  electromagnet,  E  (fig.  412),  which 
whenever  the  current  is  transmitted,  acts  attractively  on  an  armature 
of  soft  iron,  ;#,  fixed  at  the  end  of  a  lever,  mn,  movable  about  an 
axis;  when  the  current  is  open,  the  lever  is  raised  by  a  spring,  R.  At 


Fig.  412. 

the  other  end  of  the  lever  there  is  a  pencil  x,  which  writes  the 
signals.  For  this  purpose  a  long  band  of  strong  paper,  ab,  rolled 
round  a  drum,  S  (figs.  409  and  410),  passes  between  two  copper 
rollers  with  a  rough  surface,  turning  in  contrary  directions.  Drawn 
in  the  direction  of  the  arrows,  the  band  of  paper  becomes  rolled  on 


-488] 


Telegraph  Alphabet. 


513 


a  second  drum,  O,  which  is  turned  by  hand.  A  clockwork  motion 
placed  in  the  box,  V,  works  the  rollers,  between  which  the  band  of 
paper  passes. 

The  paper  being  thus  set  in  motion,  whenever  the  electromagnet 
works,  the  point,  x9  strikes  the  paper,  and,  without  perforating  it, 
produces  an  indentation,  the  shape  of  which  depends  on  the  time 
during  which  the  point  is  in  contact  with  the  paper.  If  it  only 
strikes  it  instantaneously,  it  makes  a  dot  (.)  ;  but  if  the  contact  is  of 
any  greater  duration  a  line  or  dash  of  corresponding  length  is  pro- 
duced. Hence,  by  varying  the  length  of  contact  of  the  transmitting 
key  at  one  station,  a  combination  of  dots  or  dashes  may  be  produced 
at  another  station,  and  it  is  only  necessary  to  give  a  definite  meaning 
to  these  combinations. 

This  is  effected  as  follows  in  Morse's  alphabet  : 


SINGLE 

SlNGnj 

PRINTING. 

NEEDLE. 

FEINTING. 

NEEDLE. 

A       

x/ 

N       

A 

B        

AV 

0      

I/I 

C       -- 

AA 

P      

Jk 

D      

AN 

Q         

IIJ 

E      - 

N 

R       

vA 

F      

\\A 

S 

SN< 

G     

/A 

T        — 

/ 

H     

NNNN 

TJ       

\\/ 

I      -- 

SN 

V        

NNN/ 

J    

N/// 

W        

Jl 

K      

IJ 

X       

As/ 

L      

J* 

Y       

A// 

M     

11 

Z        

/As 

Fig-  413- 

The  other  signals  are  those  of  the  single  needle  instrument 
(483).     The  signal  V   denotes  a  deflection  of  the  top  of  the  vertical 

L  L 


514 


Electric  Telegraph 


[488- 


needle  to  the  left,  and  the  signal  /  to  the  right.  They  correspond 
respectively  to  the  dot  and  dash  of  the  Morse  alphabet. 

Any  one  present  while  a  message  is  being  received  at  a  telegraph 
station,  is  astonished  at  the  promptitude  and  accuracy  with  which 
signals  are  read  and  transmitted  by  the  operators.  These  acquire 
such  skill  that  they  can  read  a  message  by  the  sounds  which  the  arma- 
ture makes  in  striking  against  the  electromagnet  of  the  indicator. 

Based  on  this  fact  a  form  of  instrument  invented  in  America  has 
come  into  use  for  the  purpose  of  reading  by  sound.  The  sounder, 
as  it  is  called,  is  essentially  a  small  electromagnet  on  an  ebonite 
base,  resembling  the  relay  in  fig.  416.  The  armature  is  attached  to 
one  end  of  a  lever,  and  is  kept  at  a  certain  distance  from  the  electro- 
magnet by  a  spring.  When  the  current  passes  the  armature  is  at- 
tracted against  the  electromagnet,  with  a  sharp  click,  and  when  the 
current  ceases  it  is  withdrawn  by  the  spring.  Hence  the  interval 
between  the  sounds  is  of  longer  or  shorter  duration  according  to 
the  will  of  the  sounder,  and  thus  in  effect  a  series  of  short  and  long 
sounds  can  be  produced  which  correspond  to  the  dots  and  dashes 
of  the  Morse  alphabet. 

489.  Improvements  in  Morse's  telegraph. — In  the  apparatus 
just  described,  the  indentations  on  the  paper  only  give  indistinct 
dots  and  dashes,  unless  the  current  transmitted  be  very  powerful. 


To  get  rid  of  this  inconvenience,  and  to  expend  less  force,  the 
apparatus  has  been  modified  so  that  the  signals  can  be  traced  in 
ink.  With  this  view,  all  the  other  parts  being  the  same,  the  follow- 
ing arrangement  is  made  : 


-491]  Electrical  A  larnm.  5 1 5 

A  roller,  a,  fig.  414,  covered  with  flannel,  is  moistened  with  a 
suitable  ink.  Above  the  roller,  and  in  contact  with  it,  is  an  endless 
band,  p  h,  rolled  on  two  pulleys,  0, 0',  which  are  turned  by  the  clock- 
work motion  which  moves  the  paper.  This  is  kept  by  a  roller, 
£,  very  near  the  chain,  but  not  touching  it.  That  being  premised, 
whenever  the  current  passes  in  the  electromagnet,  the  armature,  A, 
is  attracted,  the  arm  of  the  lever  k,  is  depressed,  and  a  pin,  2,  at  its 
end  rests  on  the  band,  and  places  it  in  contact  with  the  paper. 
The  band  depositing  the  ink  which  it  has  taken  from  the  roller, 
makes  on  the  paper  as  it  moves  along,  a  dot  or  a  dash,  according 
to  the  length  of  time  the  current  passes,  and  which  dots  and  dashes 
have  the  same  meaning  as  above. 

490.  XiigHtningr  conductor.— Besides  the  parts  of  the  telegraph 
already  described  there  are  three  of  which  mention  must  be  made ; 
the  lightning  conductor,  the  alarum,  and  the  relay.  • 

The  influence  of  storm  clouds  in  decomposing  the  natural  elec- 
tricity of  the  wire,  often  produces  sufficient  tension,  not  merely  to 
interfere  with  the  transmission  of  the  despatches,  but  also  to  pro- 
duce dangerous  discharges.  The  lightning  conductor  is  designed 
to  remedy  these  inconveniences. 

Represented  at  M  in  figs.  409  and  410,  it  consists  of  a  vertical 
stand  on  which  are  two  copper  plates,  indented  like  a  saw,  and 
arranged  so  that  the  teeth  are  near  each  other  but  do  not  touch. 
One  of  these  plates  is  connected  with  the  earth,  the  other  with  the 
line  wire.  Hence,  when,  by  the  inductive  action  of  a  storm  cloud, 
electricity  accumulates  in  wires  and  in  the  apparatus,  it  escapes  by 
the  points  to  the  plate  which  is  connected  with  the  ground,  and 
thus  all  danger  from  a  discharge  is  avoided. 

491.  Electrical  alarum. — The  electrical  alarum  is  intended  to 
warn  the  receiving  station  that  a  despatch  is  about   to  be  sent. 
Represented  in  fig.  415,  it  consists  of  a  board  on  which  is  fixed  an 
electromagnet  by  means  of  a  piece  of  brass,  E.     The  current  from 
the  line  arriving  by  a  binding  screw,  ;;/,  passes  to  the  wire  of  the 
electromagnet,  thence  into  the  armature,  a,  into  a  steel  spring,  c, 
which  presses  against  the  armature,  and  ultimately  emerges  by  a 
second  terminal,  n. 

Thus,  whenever  the  current  of  the  line  wire  reaches  the  electro- 
magnet, the  armature,  a,  is  attracted,  and  a  clapper,  P,  fixed  to  this 
armature,  strikes  against  "a  bell,  T,  and  makes  it  sound.  The 
moment  the  clapper  strikes,  as  the  armature  is  no  longer  in  contact 
with  the  spring,  C,  the  current  is  open,  the  electromagnet  no  longer 

LL  2 


516 


Electric  Telegraph. 


[491- 


attracts, ^.nd  the  armature  reverts  to  its  original  position  by  the 
action  of  a  spring,  e,  to  which  it  is  fixed. 

The  current  being  closed  afresh,  a  second  attraction  takes  place, 

a^d  so  on  until  the  telegraph 
clerk,  thus  warned,  lets  the  cur- 
rent pass  directly  into  the  indi- 
cator without  passing  through 
the  alarum.  This  he  accom- 
plishes by  means  of  an  instru- 
ment called  the  shunt. 

Relay.  In  describing  the  re- 
ceiver we  have  assumed  that  the 
current  «pf  the  line*  coming  by 
the  wire,  C  (fig.  416),  entered 
directly  into  the  electromagnet, 
and  worked  the  armature,  A, 
producing  a  despatch  ;  but  when 
the  current  has  to  traverse  a  dis- 
tance of  a  few  miles,  owing  to 
the  resistance  of  the  wire  and 
the  losses  of  insulation,  its  in- 
tensity is  diminished  so  greatly 
that  it  cannot  act  upon  the  elec- 
tromagnet with  sufficient  force 

to  print  a  despatch.  Hence  it  is  necessary  to  have  recourse  to  a  relay, 
that  is,  to  an  auxiliary  electromagnet,  which  is  still  traversed  by  the 
current  of  the  line,  but  which  serves  to  introduce  into  the  communi- 
cator the  current  of  a  local  battery  of  four  or  five  elements  placed  at 
the  station,  and  only  used  to  print  the  signals  transmitted  by  the  wire. 
For  this  purpose  the  current  from  the  line  entering  the  relay  by 
the  binding  screw,  L  (fig.  416),  passes  into  an  electromagnet,  E, 
whence  it  passes  into  the  earth  by  the  binding  screw,  T.  Now,  each 
time  that  the  current  of  the  line  passes  into  the  relay,  the  electro- 
magnet attracts  an  armature,  A,  fixed  at  the  bottom  of  a  vertical 
lever,  /,  which  oscillates  about  a  horizontal  axis. 

At  each  oscillation  the  top  of  the  lever,/,  strikes  against  a  button, 
#,  and  at  this  moment  the  current  of  the  local  battery  which  enters 
by  the  binding  screw,  r,  ascends  the  column,  m,  passes  into  the 
lever,/,  descends  by  the  rod,  0,  which  transmits  it  to  the  binding 
screw,  T  :  thence  it  enters  the  electromagnet  of  the  indicator,  whence 
it  emerges  by  the  wire,  Z,  to  return  to  the  local  battery  from  which 
it  started.  Then  when  the  current  of  the  line  is  open,  the  electro- 


-492] 


Electromagnetic  Machines. 


517 


magnet  of  the  relay  does  not  act,  and  the  lever,  /,  drawn  by  a 
spring,  r,  leaves  the  bottom,  ;/,  as  shown  in  the  drawing,  and  the 
local  current  no  longer  passes.  Thus  the  relay  transmits  to  the 


Fig.  416. 

indicator  exactly  the  same  phases  of  passage  and  intermittence  as 
those  effected  by  the  key  in  the  station  which  sends  the  despatch. 

492.  Electromagnetic  machines. —  Many  physicists  have  at- 
tempted to  utilise  the  attractive  force  of  electromagnets  as  a  motive 
power.  M.  Jacobi,  of  St.  Petersburg,  appears  to  have  been  the  first 
to  construct  a  machine  of  this  kind,  with  which,  in  1838,  he  moved 
on  the  Neva  a  small  boat  containing  twelve  persons.  Since  that 
time  the  construction  of  these  machines  has  been  materially  modi- 
fied ;  but  in  all  the  expense  of  zinc  and  acids  which  they  use  far 
exceeds  that  of  steam  engines  of  the  same  force.  Until  some 
cheaper  source  of  electricity  shall  have  been  discovered  there  is  no 
expectation  that  they  can  be  applied  at  all  advantageously. 

Fig.  418  represents  an  electromagnetic  machine  constructed  by 
Froment.  It  consists  of  four  electromagnets  acting  in  two  couples, 
on  two  pieces  of  soft  iron,  P,  only  one  of  which  is  seen  in  the 
figure.  This  piece,  attracted  by  the  electromagnets,  EF,  transmits 
the  motion  by  means  of  a  connecting  rod  to  a  crank  m  fixed  at 
the  end  of  a  horizontal  axis.  To  this  is  fixed  a  fly-wheel  like  that 
of  a  steam  engine,  which  is  intended  to  regulate  the  rotatory  'mo- 
tion. On  this  axis  also  is  a  piece  of  metal,  «,  of  a  greater  diameter, 
the  action  of  which  will  be  described  presently. 

The  current  of  the  battery,  entering  at  A,  passes  into  a  cast-iron 
base,  B,  then  by  various  metallic  connections  it  reaches  the  metal 
piece,  n.  Thence  the  current  ought  to  pass  alternately  to  the  first 


Electromagnetic  Machines. 


[492- 


-493]  Induction  by  Currents.  519 

couple  of  electromagnets,  EF,  and  then  to  the  second,  ef.  In  order 
to  understand  how  this  alteration  in  the  path  of  the  current  is  ef- 
fected, let  us  refer  to  fig.  417  on  the  right  of  the  picture,  which  re- 
presents a  section  of  the  piece, n,  and  its  accessories.  On  this  piece 
is  a  projection,  e,  which  is  called  a  cam,  and  which,  during  a  com- 
plete turn,  successively  touches  two  springs,  a  and  b  ;  these  are  in- 
tended to  transmit  to  the  electromagnets  the  current,  the  direction 
of  which  is  indicated  by  the  unbarbed  arrows  ;  the  barbed  arrows 
do  not  show  the  direction  of  the  current  but  the  direction  of  the 
motion  of  the  various  pieces  of  the  machine. 

These  details  being  known,  it  will  be  seen  that  the  current  passes 
alternately  into  two  springs,  #"and  b,  and  from  thence  into,  the 
two  systems  of  electromagnets,  EF  and  ef:  the  piece  P  is  first  of 
all  attracted,  then  a  similar  one,  which  is  placed  at  the  other,  end  of 
the  axis  of  the  fly-wheel.  There  is  thus  produced  a  continuous  cir- 
culating motion,  which  is  transmitted  by  an  endless  band  to  a 
system  01  wheel  work,  which  works  two  lifting  pumps. 


CHAPTER   XII. 

INDUCTION   BY  CURRENTS. 

493.  Induction  by  currents. — We  have  already  seen  (398) 
that  by  the  term  induction  is  meant  the  action  which  electrified 
bodies  exert  at  a  distance  on  bodies  in  the  natural  state.  Hitherto 
we  have  only  had  to  deal  with  electrostatical  induction ;  we  shall 
now  see  that  dynamical  electricity  produces  analogous  effects. 

Faraday  discovered  this  class  of  phenomena  in  1832,  and  he  gave 
the  name  of  currents  of  induction,  or  induced  currents,  to  instanta- 
neous currents  developed  in  metallic  conductors  under  the  influ- 
ence of  metallic  conductors  traversed  by  electric  currents,  or  by  the 
influence  of  powerful  magnets,  or  even  by  the  magnetic  action  of 
the  earth  ;  and  the  currents  which  give  rise  to  them  he  called 
inducing  currents. 

The  inductive  action  of  currents  at  the  moment  of  opening  or 
closing  may  be  shown  by  means  of  a  coil  with  two  wires.  This 
consists  (fig.  419)  of  a  hollow  cylinder  of  wood  or  of  cardboard,  on 
which  a  quantity  of  stout  silk-covered  copper  wire  is  coiled  ;  on  this 


520 


Induction. 


[493- 


is  coiled  a  considerably  greater  length  of  fine  copper  wire,  also  in- 
sulated by  being  covered  with  silk.  This  latter  coil,  which  is  called 
the  secondary  coil,  is  connected  by  its  ends  with  two  binding 
screws,  a,  b,  from  which  wires  pass  to  a  galvanometer  G.  while  the 
thicker  wire,  the  primary  coil,  is  connected  by  its  extremities  with 
two  binding  screws,  c  and  d.  One  of  these,  d,  being  connected 
with  one  pole  of  a  battery,  when  a  wire  from  the  other  pole  is  con- 
nected with  c,  the  current  passes  in  a  primary  coil,  and  in  this 
alone.  The  following  phenomena  are  then  observed  : 

i.  At  the  moment  at  which  the  thick  wire  is  traversed  by  the 
current,  the  galvanometer,  by  the  deflection  of  the  needle,  indicates 
the  existence  in  the  secondary  coil  bf  a  current  inverse  to  that  in 
the  primary  coil,  that  is,  in  the  contrary  direction  :  this  is  only  in- 
stantaneous, for  the  needle  immediately  reverts  to  zero,  and  remains 
so  as  long  as  the  inducing  current  passes  through  cd. 


Fig.  419. 

ii.  At  the  moment  at  which  the  current  is  opened,  that  is,  when 
the  wire,  cd,  ceases  to  be  traversed  by  a  current,  there  is  again  pro- 
duced in  the  wire,  ab,  an  induced  current  instantaneous  like  the 
first,  but  direct,  that  is,  in  the  same  direction  as  the  inducing  current. 

494.  Induction  by  magnets  and  by  the  action  of  the  earth. — 
It  has  been  seen  that  the  influence  of  a  current  magnetises  a  steel 
bar  ;  in  like  manner  a  magnet  can'  produce  induced  electrical 
currents  in  metallic  circuits.  Faraday  showed  this  by  means  of 
a  coil  with  a  single  wire  of  200  to  300  yards  in  length.  The  two 
extremities  of  the  wire  being  connected  with  the  galvanometer, 
as  shown  in  fig.  420,  a  strongly  magnetised  bar  is  suddenly  in- 
serted in  the  bobbin,  and  the  following  phenomena  are  observed  : 


-494] 


Induction  by  Magnets. 


521 


i.  At  the  moment  at  which  the  magnet  is  introduced,  the  galva- 
nometer indicates  in  the  wire  the  existence  of  a  current,  the  direc- 
tion of  which  is  opposed  to  that  which  circulates  round  the  magnet, 
considering  the  latter  as  a  solenoid  on  Ampere's  theory  (480). 

ii.  The  needle  soon  returns  to  zero,  and  remains  there  as  long  as 
the  magnet  is  in  the  coil  ;  when  it  is  withdrawn,  the  needle  of  the 
galvanometer,  which  has  returned  to  zero,  indicates  the  existence 
of  a  direct  current. 

The  inductive  action  of  magnets  may  also  be  illustrated  by  the 
following  experiment  :  A  bar  of  soft  iron  is  placed  in  the  above 
bobbin  and  a  strong  magnet  suddenly  brought  in  contact  with  it  ; 
the  needle  of  the  galvanometer  is  reflected,  but  returns  to  zero 
when  the  magnet  is  stationary,  and  is  deflected  in  the  opposite 
direction  when  it  is  removed.  The  induction  is  here  produced  by 


Fig.   420. 


the  magnetisation  of  the  soft  iron  bar  in  the  interior  of  the  bobbin 
under  the  influence  of  the  magnet. 

Faraday  discovered  that  terrestrial  magnetism  can  develope  in- 
duced currents  in  metallic  bodies  in  motion  ;  that  it  acts  like  a 
powerful  magnet  placed  in  the  interior  of  the  earth  in  the  direction 
of  the  dipping  needle,  or  according  to  the  theory  of  Ampere,  like  a 
series  of  electrical  currents  directed  from  east  to  west  parallel  to 
the  magnetic  equator.  He  first  proved  this  by  placing  a  long 


522  Induction.  [494- 

helix  of  copper  wire  covered  with  silk  in  the  plane  of  the  magnetic 
meridian  parallel  to  the  dipping  needle  ;  by  turning  this  helix 
through  a  semicircle  round  an  axis  in  its  middle,  perpendicular  to 
its  length,  he  observed  that  at  each  turn  a  galvanometer,  connected 
with  the  two  ends  of  the  helix,  was  deflected. 

495.  Properties  of  induced  currents. — Notwithstanding  their 
instantaneous  character,  it  appears  mainly  from  the  experiments  of 
Faraday  their  discoverer,  that  induced  currents  have  all  the  pro- 
perties of  ordinary   currents.     They  produce  violent  physiological 
luminous,  calorific,  and  chemical  effects,  and  finally  give  rise  to 
new  induced  currents.     They  also  deflect  the  magnetic  needle,  and 
magnetise  steel  bars  when  they  are  passed  through  a  copper  wire 
coiled  in  a  helix  round  the  bars. 

The  intensity  of  the  shock  produced  by  induced  currents  renders 
their  effects  comparable  to  those  of  electricity  in  a  state  of  tension. 
But  as  they  act  on  the  galvanometer  the  electricity  is  present,  both 
in  a  state  of  tension  and  in  the  dynamical  condition. 

These  phenomena  of  induction  currents  are  well  seen  in  Ruhm- 
korff's  coil,  which  we  shall  now  describe. 

496.  RuhmkorfPs  coil.—  This  is  an  arrangement  for  producing 
induced  currents,  in  which  a  current  is  induced  by  the  action  of  an 
electric  current,  whose  circuit  is  alternately  opened  and  closed  in 
rapid  succession.     These  instruments,  known  as  inductoriums,  or 
induction  coils,  present  considerable  variety  in  their  construction, 
but  all  consist  essentially  of  a  hollow  cylinder  in  which  is  a  bar  of 


Fig.  421. 

soft  iron,  or  bundle  of  iron  wires,  with  two  helices  coiled  round  it, 
one  connected  with  the  poles  of  a  battery,  the  current  of  which  is 


-496] 


Ruhmkorjf's  Coil. 


523 


alternately  opened  and  closed  by  a  self-acting  arrangement,  and 
the  other  serving  for  the  development  of  the  induced  current.  By 
means  of  these  apparatus,  with  a  current  of.  three  or  four  Grove's 
cells,  physical,  chemical,  and  physiological  effects  are  produced 
equal  to  and  superior  to  those  obtainable  with  electrical  machines 
and  even  the  most  powerful  Leyden  batteries. 

Of  all  the  forms  those  constructed  by  Ruhmkorff  in  Paris,  and  by 
Ladd  and  Apps  in  this  country,  are  the  most  powerful.  Fig.  421 
is  a  representation  of  one,  the  coil  of  which  is  about  14  inches  in 
length.  T\\e  pi'imary  or  inducing  wire  is  of  copper,  and  is  about 
2  mm.  in  diameter,  and  14  or  1 5  yards  in  length.  It  is  coiled  directly 
on  a  cylinder  of  cardboard,  which  forms  the  nucleus  of  the  appa 
ratus,  and  is  enclosed  in  an  insulating  cylinder  of  glass,  or  of 
ebonite.  On  these  is  coiled  the  secondary  or  induced  wire, 
which  is  also  of  copper,  and  is  about  |mm.  in  diameter.  A  great 
point  of  these  apparatus  is  the  insulation.  The  wires  are  not 
merely  insulated  by  being  in  the  first  case  covered  with  silk,  but 
each  individual  coil  is  separated  from  the  rest  by  a  layer  of  melted 
shellac.  The  length  of  the  secondary  wire  varies  greatly  ;  in  some 
of  the  largest  sizes  it  is  as  much  as  several  miles.  With  these  great 
lengths  the  wire  is  thinner,  about  |mm. 

The  following  is  the  working  of  the  apparatus.  The  current 
arriving  by  the  wire,  P,  at  a  binding  screw,  a,  passes  thence  into  the 
commutator,  C  (fig.  421)  ;  thence  by  the  binding  screw,  b,  it  enters 
the  primary  wire,  where  it  acts  inductively  on  the  secondary  wire  ; 
having  traversed  the  primary  wire  it  emerges  by  the  wire,  s  (fig.  422). 
Following  the  direction  of  the 
arrows,  it  will  be  seen  that 
the  current  ascends  in  the 
binding  screw,  z,  reaches  an 
oscillating  piece  of  iron,  o, 
called  the  hammer,  descends 
by  the  anvil,  /i,  and  passes 
into  a  copper  plate,  K,  which 
takes  it  to  the  commutator, 
C.  It  goes  from  there  to 
the  binding  screw,  c,  and 
finally  to  the  negative  pole 
of  the  battery  by  the  wire,  N.  Fi  • 

The  current  in  the  primary 
wire  only  acts  inductively  on  the  secondary  wire  (493),  when  it 


524  Induction.  [496- 

opens  or  closes,  and  hence  it  must  be  constantly  interrupted.  This 
is  effected  by  means  of  the  oscillating  hammer,  o,  omitted  in  figure 
421,  but  represented  on  a  larger  scale  in  fig.  422.  In  the  centre  of 
the  bobbin  is  a  bundle  of  soft  iron  wires,  forming  together  a  cylin- 
der a  little  larger  than  the  bobbin,  and  thus  projecting  at  the  end 
as  seen  at  A.  When  the  current  passes  in  the  primary  wire,  this 
hammer,  <?,  is  attracted  ;  but  immediately,  there  being  no  contact 
between  o  and  h,  the  current  is  broken,  the  magnetisation  ceases, 
and  the  hammer  falls  ;  the  current  again  passing,  the  same  series 
of  phenomena  recommences,  so  that  the  hammer  oscillates  with 
great  rapidity. 

In  proportion  as  the  current  passes  thus  intermittently  in  the 
primary  wire  of  the  bobbin,  at  each  interruption  an  induced  current, 
alternately  direct  and  inverse,  is  produced  in  the  secondary  wire. 
But  as  this  is  perfectly  insulated,  the  current  acquires  such  an 
intensity  as  to  produce  very  powerful  effects.  Fizeau  increased 
this  intensity  by  interposing  a  condenser  in  the  induced  circuit.  As 
constructed  by  Ruhmkorff,  for  his  largest  apparatus,  it  consists 
of  150  sheets  of  tinfoil  about  18  inches  square  ;  these  sheets  being 
joined  are  coiled  on  two  sides  of  a  sheet  of  oiled  silk,  which  insulates 
them,  forming  thus  two  armatures  ;  they  are  then  coiled  several 
times  round  each  other,  so  that  the  whole  can  be  placed  below  the 
helix  in  the  base  of  the  apparatus.  One  of  these  armatures,  the 
positive,  is  connected  with  the  binding  screw,  /,  which  receives  the 
current  on  emerging  from  the  bobbin  ;  and  the  other,  the  negative, 
is  connected  with  the  binding  screw,  m,  which  communicates  by 
the  plate,  K,  with  the  commutator,  C,  and  with  the  battery. 

497.  Effects  produced  by  RuhmkorfTs  coil. — The  high  degree 
of  tension  which  the  electricity  of  induction  coil  machines  possesses 
has  long  been  known,  and  many  luminous  and  calorific  effects 
have  been  obtained  by  their  means.  But  it  is  only  since  the 
improvements  which  Ruhmkorff  has  introduced  into  his  coil,  that 
it  has  been  possible  to  utilise  all  the  tension  of  induced  currents, 
and  to  show  that  these  currents  possess  the  properties  of  statical 
as  well  as  dynamical  electricity. 

Induced  currents  are  produced  in  the  coil  at  each  opening  and 
breaking  of  contact.  But  these  currents  are  not  equal  either  in 
duration  or  in  tension.  The  direct  current,  or  that  on  opening,  is 
of  shorter  duration,  but  more  tension  ;  that  of  closing  of  longer 
duration  but  less  tension.  Hence  if  the  two  ends  P,  and  P',  of  the 
fine  wire  (figs.  421  and  422)  are  connected,  as  there  are  two  equal 


-497]  Effects  of  Ruhmkorff's  Coil  '525 

and  contrary  quantities  of  electricity  in  the  wire  the  two  currents 
neutralise  each  other.  If  a  galvanometer  is  placed  in  the  circuit, 
only  a  very  feeble  deflection  is  produced  in  the  direction  of  the 
direct  current.  This  is  not  the  case  if  the  two  extremities,  P  and 
P',  of  the  wire  are  separated.  As  the  resistance  of  the  air  is 
then  opposed  to  the  passage  of  the  currents,  that  which  has  most 
tension,  that  is,  the  direct  one,  passes  in  excess,  and  the  more  so 
the  greater  the  distance  of  P  and  P'  up  to  a  certain  limit  at  which 
neither  pass.  There  are  then  at  P  and  P'  nothing  but  tensions 
alternately  in  contrary  directions. 

The  effects  of  the  coil,  like  those  of  the  battery,  may.be  classed 
under  the  heads  physiological,  chemical,  calorific,  luminous,  me- 
chanical ;  they  have  this  difference,  that  they  are  enormously  more 
intense. 

The  physiological  effects  of  RuhmkorfFs  coil  are  very  powerful ; 
in  fact,  the  shocks  are  so  violent  that  many  experimenters  have 
been  suddenly  prostrated  by  them.  A  rabbit  may  be  killed  with 
an  induction  current  arising  from  two  of  Bunsen's  elements,  and  a 
somewhat  larger  number  of  couples  would  kill  a  man. 

The  calorific  effects  are  also  easily  observed  ;  it  is  simply  neces- 
sary to  interpose  a  very  fine  iron  wire  between  the  two  ends,  P  and 
P',  of  the  induced  wire  ;  this  iron  wire  is  immediately  melted,  and 
burns  with  a  bright  light.  The  spark  of  the  RuhmkorfFs  coil  is 
used  to  fire  mines  in  military  and  mining  operations. 

The  chemical  effects  are  very  varied,  inasmuch  as  the  apparatus 
produces  both  dynamical  electricity  and  electricity  in  a  high  state 
of  tension.  Thus,  according  to  the  shape  and  distance  of  the 
platinum  electrodes  immersed  in  water,  and  to  the  degree  of  acidu- 
lation  of  the  water,  either  luminous  effects  may  be  produced  in 
water  without  decomposition,  or  the  water  may  be  decomposed  and 
the  mixed  gases  disengaged  at  the  two  poles,  or  the  decomposition 
may  take  place,  and  the  mixed  gases  separate  either  at  a  single 
pole  or  at  both  poles. 

The  luminous  effects  of  RuhmkorrFs  coil  are  also  very  remark- 
able and  vary  according  as  they  take  place  in  air,  in  vacuo,  or  in 
very  rarefied  vapours.  In  air  the  coil  produces  a  very  bright  loud 
spark,  which,  with  the  largest-sized  coils,  has  a  length  of  eighteen 
inches.  In  vacuo  the  effects  are  also  remarkable.  The  experiment 
is  made  by  connecting  the  two  wires  of  the  coil,  P  and  P',  with  the 
two  rods  of  the  electrical  egg  (fig.  351),  used  for  producing  in 
vacuo  the  luminous  effects  of  the  electrical  machine.  A  vacuum 


526'  Induction.  [497- 

having  been  produced,  a  beautiful  luminous  trail  is  produced  from 
one  knob  to  the  other,  which  is  virtually  constant,  and  has  the 
same  intensity  as  that  obtained  with  a  powerful  electrical  machine 
when  the  plate  is  turned. 

If  this  light  be  closely  observed,  it  will  be  found  that  if  some 
vapour  of  turpentine,  or  wood  spirit,  or  bisulphide  of  carbon,  have 
been  introduced  into  the  globe  before  exhaustion,  instead  of  being 
continuous,  the  light  consists  of  a  series  of  alternately  dark  and- 
bright  zones,  forming  a  pile  of  electric  light  between  the  two 
poles.  This  phenomenon  is  known  as  the  stratification  of  the 
electric  light,  and  is  due  to  the  circumstance  that  the  current  is  dis- 
continuous. 

The  brilliancy  and  beauty  of  the  stratification  of  the  electric 
light  are  most  remarkable  when  the  discharge  of  the  Ruhmkorff's, 
coil  takes  place  in  glass  tubes  containing  a  highly  rarefied  vapour 
or  gas.  These  phenomena,  which  have  been  investigated  by  Mas- 
son,  Grove,  Gassiot,  Pliicker,  etc.,  are  produced  by  means  of  sealed 
glass  tubes  first  constructed  by  Geissler,  of  Bonn.  These  tubes 
are  filled  with  different  gases  or  vapours,  and  are  then  exhausted. 
At  the  ends  of  the  tubes  two  platinum  wires  are  soldered  into  the 
glass.  See  figs,  on  the  right  and  left  in  the  coloured  plate. 

When  the  two  platinum  wires  are  connected  with  the  ends  of  a 
RuhmkorfFs  coil,  magnificent  lustrous  striae,  separated  by  dark 
bands,  are  produced  all  through  the  tube.  These  striae  vary  in 
shape,  colour,  and  lustre  with  the  degree  of  the  vacuum,  the  nature 
of  the  gas  or  vapour,  and  the  dimensions  of  the  tube.  The  pheno- 
menon has  occasionally  a  still  more  brilliant  aspect  from  the  fluor- 
escence which  the  electric  discharge  excites  in  the  glass. 

The  figure  on  the  left  represents  the  appearance  presented  by 
hydrogen  ;  in  the  bulbs  the  light  is  a  pale  lavender  blue,  in  the 
capillary  parts  it  is  red. 

In  carbonic  acid  the  colour  is  greenish,  and  the  striae  have  not 
the  same  shape  as  in  hydrogen  ;  in  nitrogen,  as  represented  in  the 
figure  on  the  right,  the  light  is  reddish  violet.  In  chlorine  the 
colour  is  reddish  violet  in  the  wide  part  of  the  tube,  and  in  very 
narrow  tubes  green. 

Mechanical  effects.  By  means  of  RuhmkorfFs  coil  mechanical 
effects  can  also  be  produced,  so  powerful  that,  with  the  largest  ap- 
paratus, glass  plates  two  inches  thick  have  been  perforated.  The 
result,  however,  is  not  obtained  by  a  single  discharge,  but  by  several 
successive  discharges. 


-498]     Rotation  of  Induced  Currents  by  Magnets.       527 

The  experiment  is  arranged  as  shown  in  fig.  423.  The  two  poles 
of  the  induced  current  correspond  to  the  binding  screws,  a  and  b ; 
by  means  of  a  copper  wire,  /,  the  pole,  a,  is  connected  with  the  lower 
part  of  an  apparatus  for  piercing  glass  like  that  already  described 
(fig.  369),  the  other  pole  is  attached  to  the  upper  conductor  by  a 
wire,  d.  This  conductor  is  insulated  in  a  large  glass  tube,  r,  filled 
with  shellac,  which  is  run  in  while  in  a  state  of  fusion.  Between  the 
two  conductors  is  the  glass  to  be  perforated,  V.  When  this  pre- 


Fig.  423. 

sents  too  great  a  resistance,  there  is  danger  lest  the  spark  pass  in 
the  coil  itself,  perforating  the  insulated  layer  which  separates  the 
wire,  and  then  the  coil  is  destroyed.  To  avoid  this,  two  wires,  e 
and  c,  connect  the  poles  of  the  coil  with  two  metal  rods,  m  and  «, 
whose  distance  from  each  other  can  be  regulated.  If  then  the  spark 
cannot  penetrate  through  the  glass,  it  bursts  across  with  a  bright 
spark  and  a  loud  report,  and  the  coil  is  not  injured. 

498.  Rotation  of  induced  currents  by  magnets. — De  la  Rive 
has  recently  devised  an  experiment  which  shows  in  a  most  in- 
genious and  beautiful  manner  that  magnets  act  on  the  light  in 
Geissler's  tubes  in  accordance  with  the  laws  with  which  they  act  on 
any  other  movable  conductor. 

On  the  iron  core  of  an  electromagnet,  M  (see  coloured  plate)  is 
a  soft  iron  rod  terminated  at  the  top  by  an  iron  plate  ;  this  rod,  with 
the  exception  of  the  top,  a,  is  inserted  in  a  very  carefully  insulated 
glass  tube.  The  binding  screw,  /£,  is  in  conducting  communication 
with  this  iron  rod.  The  whole  of  the  upper  part  of  this  arrange- 


528  Thermoelectricity.  [498- 

ment  is  fitted  into  the  tubulure  of  an  electrical  egg ;  with  which 
the  brass  tubulure,  dd,  which  holds  the  glass  tube  is  in  conducting 
communication.  By  the  stopcock  at  the  top  the  electrical  egg  can 
be  exhausted,  and  a  few  drops  of  alcohol  are  then  introduced. 

If  now  the  wires  from  a  Ruhmkorff ' s  coil  are  connected  with 
the  binding  screws,  h  and  /£,  but  without  at  the  same  time  exciting 
the  electromagnet,  a  more  or  less  irregular  luminous  sheaf  passes 
from  the  plate,  a,  to  the  ring,  dd. 

But  if  a  voltaic  current  passes  into  the  electromagnet  the  phe- 
nomenon is  different  ;  instead  of  starting  from  different  points  of 
the  upper  surface  and  the  ring,  the  light  is  condensed  and  emits  a 
single  luminous  arc.  Further,  and  this  is  the  most  remarkable  part 
of  the  experiment,  this  arc  turns  slowly  round  the  magnetised 
cylinder,  sometimes  in  one  direction,  and  sometimes  in  another, 
according  to  the  direction  of  the  induced  current,  or  the  direction  of 
the  magnetisation  evoked  in  the  core.  As  soon  as  the  magnetisation 
ceases  the  luminous  phenomenon  reverts  to  its  original  appearance. 

This  experiment  is  remarkable  as  having  been  devised  a  priori 
by  De  la  Rive  to  explain,  by  the  influence  of  terrestrial  magnetism, 
a  kind  of  rotatory  motion  from  east  to  west,  observed  in  the  aurora 
borealis.  The  rotation  of  the  luminous  arc  in  the  above  experiment 
can  evidently  be  referred  to  the  rotation  of  currents  by  magnets. 


CHAPTER  XIII. 
THERMOELECTRIC  CURRENTS.     . 

499.  Thermoelectricity. — In  1821,  Professor  Seebeck,  in  Berlin, 
found  that  by  heating  one  of  the  junctions  of  a  metallic  circuit, 
consisting  of  two  metals  soldered  together,  an  electric  current  was 
produced.  This  phenomenon  may  be  shown  by  means  of  the 
apparatus  represented  in  fig.  424,  which  consists  of  a  plate  of 
copper,  mn,  the  ends  of  which  are  bent  and  soldered  to  a  plate  of 
bismuth,  op.  In  the  interior  of  the  circuit  is  a  magnetic  needle,  a, 
moving  on  a  pivot.  When  the  apparatus  is  placed  in  the  magnetic 
meridian,  and  one  of  the  solderings  gently  heated,  as  shown  in  the 
figure,  the  needle  is  deflected  in  a  manner  which  indicates  the 
passage  of  a  current  from  n  to  m,  that  is,  from  the  heated  to  the 
cool  junction  in  the  copper.  If,  instead  of  heating  the  junction,  ;/, 


-500] 


Thermoelectric  Scries. 


529 


it  is  cooled  by  ice,  or  by  placing  upon  it  cotton  wool  moistened  with 
ether,  the  other  junction  remaining  at  the  ordinary  temperature, 
a  current  is  produced,  but  in  the  opposite  direction  ;  that  is  to 


Fig.  424- 

say,  from  m  to  n.  In  both  cases  the  current  is  more  energetic  in 
proportion  as  the  difference  in  temperature  of  the  solderings  is 
greater. 

Seebeck  gave  the  name  thermoelectric  to  this  current,  and  the 
couple  which  produces  it,  to  distinguish  it  from  the  hydroelectric  or 
ordinary  voltaic  current  and  couple. 

500.  Thermoelectric  series. — If  small  bars  of  two  different 
metals  are  soldered  together  at  one  end  while  the  free  ends  are 
connected  with  the  wires  of  a  galvanometer,  and  if  now  the  point 
of  junction  of  the  two  metals  be  heated,  a  current  is  produced,  the 
direction  of  which  is  indicated  by  the  deflection  of  the  needle  of  the 
galvanometer,  figs.  426,  427.  By  experimenting  in  this  way  with  dif- 


ferent  metals,  they  may  be  formed  in  a  list  such  that  each  metal  gives 
rise  to  positive  electricity  when  associated  with  one  of  the  following, 

M  M 


530 


Thermoelectricity. 


[500- 


and  negative  electricity  with  one  of  those  that  precede  ;  that  is,  that 
in  heating  the  soldering,  the  positive  current  goes  from  the  positive 
to  the  negative  metal  across  the  soldering,  just  as  if  the  soldering 
represented  the  liquid  in  a  hydroelectrical  element ;  hence  out  of 
the  element,  in  the  connecting  wire  in  the  galvanometer  for  in- 
stance, the  current  goes  from  the  negative  to  the  positive  metal. 
Thus  a  couple,  bismuth-antimony,  heated  at  the  junction  would 
correspond  to  a  couple,  zinc-copper,  immersed  in  sulphuric  acid. 
Fig.  426  represents  a  battery  of  such  elements. 

Of  all  bodies,  bismuth  and  selenium  produce  the  greatest  electro- 
motive force  ;  but  from  the  expense  of  this  latter  element,  and  on 
account  of  its  low  conducting  power,  antimony  is  generally  sub- 
stituted. The  antimony  is  the  negative  metal  but  the  positive  pole, 
and  the  bismuth  the  positive  metal  but  the  negative  pole,  and  the 
current  goes  from  bismuth  to  antimony  across  the  junction. 

501.  Wobili  s  thermoelectric  pile. — Nobili  devised  a  form  ot 
thermoelectric  battery,  or  pile  as  it  is  usually  termed,  in  which 
there  are  a  large  number  of  elements  in  a  very  small  space.  For 
this  purpose  he  joined  the  couples  of  bismuth  and  antimony  in  such 
a  manner,  that  after  having  formed  a  series  of  five  couples,  as  re- 
presented in  fig.  427,  the  bismuth  from  b  was  soldered  to  the 
antimony  of  a  second  series  arranged  similarly  ;  the  last  bismuth 
of  this  to  the  antimony  of  a  third,  and  so  on  for  four  vertical  series, 
containing  together  twenty  couples,  commencing  by  antimony  finish- 
ing by  bismuth.  Thus  arranged,  the  couples  are  insulated  from 

one  another  by  means  of 
small  paper  bands  covered 
with  varnish,  and  then  en- 
closed in  a  copper  frame,  P 
(fig.  428),  so  that  only  the 
solderings  appear  at  the  two 
ends  of  the  pile.  Two  small 
copper  binding  screws,  m 
and  n,  insulated  in  an  ivory 
ring,  communicate  in  the 
interior,  one  with  the  first 
antimony,  representing  the 

positive  pole  and  the  other  with  the  last  bismuth,  representing  the 
negative  pole.  These  binding  screws  communicate  with  the  ex- 
tremities of  a  galvanometer  wire,  when  the  thermoelectric  current 
is  to  be  observed. 


Fig.  42 


Fig.  427. 


-501] 


Nobilis  Thermoelectric  Pile. 


531 


A  Nobili's  pile  in  combination  with  a  galvanometer  constitutes 
the  most  delicate  and  accurate  means  of  measuring  a  temperature. 
Such  an  arrangement  was  first  used  by  Melloni  in  his  researches  on 
the  transmission  of  radiant  heat.  The  arrangement  he  used  is 
represented  in  figure  429. 

On  a  wooden  base,  provided  with  levelling  screws,  a  graduated 
copper  rule,  about  a  yard  long,  is  fixed  edgeways.  On  this  rule  the 
various  ports  composing  the  apparatus  are  placed,  and  their  dis- 
tances can  be  fixed  by  means  of  binding  screws,  a  is  a  support  for 


Fig.  429. 

a  Locatelli's  lamp,  or  other  source  of  heat ;  F  and  E  are  screens  ; 
C  is  a  support  for  the  bodies  experimented  on,  and  m  is  a  thermo- 
electrical  battery.  Near  the  apparatus  is  a  galvanometer,  D,  which 
has  only  a  comparatively  few  turns  of  a  tolerably  thick  (imm.) 
copper  wire.  Such  galvanometers  are  called  thermomultipliers 
(473).  The  delicacy  of  this  apparatus  is  so  great  that  the  heat  of 
the  hand  is  enough,  at  a  distance  of  a  yard  from  the  pile,  to  deflect 
the  needle  of  the  galvanometer. 


M  M2 


I 


.   Erratum. 
Page  354,  line  9  from  bottom  ,for  Decomposition  read  Recomposition. 


INDEX. 


ABE 

A  BERRATION    of    refrangibility, 
«•     chromatic,      359 ;       spherical, 

361 

Absorption,  68,  137;  of  light,  302 
Absorbing  power,  213  ;  causes  which 

modify,  215 
Accelerated  motion,  17 
Accelerating  forces,  20 
Accidental  images,  356 
Achromatic  lenses,  350 
Acidometer,  105 
Acoustic  foci,  164 
Aerial  wire,  485 
Aeriform  fluids,  5 
Adhesion,  64 

Affinity,  3,  4  ;  chemical,  63 
Air,    atmospheric,   no;    hygrometric 

state  of,  273  ;  weight  of,  113 
Air-guns,  13 
Air-pump,     138,    260  ;     gauge,    139  ; 

uses  of,  140 
Alarum,  electrical,  491 
Alcarrazas,  249 
Alcohol  thermometer,  201 
Alcoholometer,     Gay-Lussac's,     107 ; 

centesimal,  107 
Alphabet,  telegraphic,  488 
Amalgam,  312 
Ampere's    rule,     471  ;     stand,     472  ; 

theories  of  magnetism,  480 
Amplitude  of  oscillation,  57 
Analysis,  spectrum,  350 
Aneroid  barometer,  135 
Angle   of  incidence,  308  ;    reflection, 

308 

Antipodes,  40 

Apparent  expansion,  225  ;  rest,  14 
Appert's  method  of  preserving  food, 

141 

Aqueous  humour,  383 
Aqueous  vapour,  247 
Arc,  voltaic,  464 


BAT 

Archimedes'  principle,  96,  102 ;  ap- 
plied to  gases,  151 

Armatures,  398,  428 

Arms  of  a  lever,  33 

Artesian  wells,  94 

Astronomical  telescopes,  364 

Atmosphere,  crushing  force  of,  115  ; 
electricity  of,  441 ;  experiments  on 
weight  of,  113  ;  heat  of,  194  ; 
height  of,  130  ;  pressure  of,  in  all 
directions,  131 

Atmospheric  pressure,  114 ;  amount 
of,  119 

Atoms,  4,  8 

Attraction,  chemical,  3,  4  ;  magnetic, 
389  ;  molecular,  4  ;  universal,  36 

Atwood's  machine,  55 

Aura,  420 

Auroras,  394 

Aurora  borealis,  447 

Aurum  musivum,  413 

Autoclaves,  267 

Axis  of  suspension,  48 


BALANCE,  48  ;   conditions  of  ac- 
curacy   and    delicacy    of,    49  ; 

Coulomb's,   405 ;    hydrostatic,    96, 

102,  103 
Balloons,  air,  152  ;  construction  and 

management  of,  153 
Band,'  endless,  261 
Barker's  mill,  79 
Barometer,  120;  cistern,  121;  Fortin's, 

122  ;    height   determined  by,    129  ; 

mean  height  of,   125  ;    precautions 

in  reference  to,  124  ;    syphon,   123  ; 

variations  of,  125 
Barometric  variations,  126,  127 
Baroscope,  151' 
Battery,    chemical    effects    of,    465  ; 

luminous  effects,  464  ;  physiological 


534 


Index, 


BAT 

effects  of,  462  ;  thermal  effects,  463  ; 
voltaic,  456 

Batteries,  constant,  458  ;  electric,  429  ; 
enfeeblement  of  the  current  in,  457. 

Beacons,  345 

Beam,  48 

Beating  reed,  187 

Beaume"s  hydrometer,  106 

Bellows,  155,  188 

Bernoulli's  laws,  189 

Berthollet's  apparatus,  136 

Binocular  vision,  385 

Biot's  experiment,  409     . 

Bladder  of  fish,  99 

Blood  globules,  8 

Bodies,  equilibrium  of,  45  ;  general 
properties  of,  6  ;  internal  constitu- 
tion of,  4 

Boiler,  steam,  267 

Boiling,  243  ;  laws  of,  244,  245 

Bologna,  Tower  of,  47 

Boyle's  law,  133 

Brahma's  press,  83 

British  units,  104 

Buffon's  burning  mirrors,  211 

Bulb  of  thermometer,  198 

Bulging  of  earth  at  the  equator,  31 

Bunsen's  battery,  460  ;  burner,  351 

Bunsen   and   Kirchhoff's   researches, 

350.  SSL  352 
Bunten's  barometer,  123 
Buoyancy  of  liquids,  80 
Burning  glasses,  344  ;  mirrors,  211 


(~* CESIUM,  353 

Vf"     Caloric,  256 

Calorific  capacity,  257 

Calorific  effects  of  the  spectrum,  348 

Calorimeter,  Rumford's,  297 

Calorimetry,  256 

Camera  obscura,  378,  379  ;  portable, 

37.8 

Capillarity,  65 
Captive  balloon,  153 
Cartesian  diver,  98 
Catoptric  telescopes,  366 
Celsius  scale,  200 
Centesimal  alcoholometer,  107 
Centigrade  scale,  200 
Centimeter,  104 
Centre  of  gravity,  41  ;  determination 

-,  of'  43 

Centrifugal  force,  29 
Charge  of  electrical  machine,  413 
Chatterton's  compound,  485 


CON 

Chemical  affinity,  63  ;  attraction,  3, 
4 ;  combinations,  296  ;  effects  of 
the  electric  battery,  464 ;  of  the 
spectrum,  348  ;  hygrometers,  273  ; 
phenomenon,  i 

Chevalvapeur,  266 

Chimes,  electrical,  418 

Chimneys,  draughts  in,  228 

Chlorine,  229 

Chords,  174  ;  vocal,  193 

Choroid,  383 

Chromatic    scale,     176  ;     aberration, 

359 

Circuit,  483 

Cirro-cumulus,  281 

Cirro-stratus,  281 

Cirrus,  281 

Cistern  barometer,  121 

Climate,  279 

Clouds,  281*;  formation  of,  282 

Coercive  force,  392 

Cohesion,  63 

Coil,  primary,  493  ;  Ruhmkorff's, 
496;  secondary,  493 

Cold,  298  ;  by  expansion  of  gases, 
299  ;  due  to  evaporation,  249 ;  noc- 
turnal radiation,  300 

Collecting  plate,  430 

Collimation,  364 

Collodion,  379 

Colour  of  heat,  216 

Colours  of  the  spectrum,  346 

Cooling,  condensation  by,  251 

Combustion,  296 

Comma,  174 

Communicator,  483 

Commutator,  483 

Compass,  inclination,  396  ;  mariners', 

395 

Compensation  pendulum,  224 

Complementary  colours,  356 

Component  forces,  24 

Compound  microscopes,  369 

Compound  musical  tones,  179 

Compound  pendulum,  57 

Compressibility,  n 

Compression  pump,  13 

Concave  mirrors,  211,  318;  focus  of, 
319  ;  formation  of  images  in,  322  ; 
'reflection  of  heat  from,  211 

Concert  pitch,  177 

Concurrent  forces,  27  ;  resultant  of,  25 

Condensation,  heat  disengaged  during, 
252  ;  hygrometers,  273  ;  by  chemi- 
cal affinity,  251  ;  by  cooling,  251  ; 
by  pressure,  251  ;  of  vapours,  251 


Index. 


535 


CON 

.Condensed  wave,  157 

Condenser,  260 

Condensers,  limit  of  charge  of,  427 

Condensing    electroscope,    430  ;     en- 

gine, 265  ;  plate,  430  ;  pump,  142 
Conductivity  of  bodies,  applications, 

221  ;  of  liquids,  219  ;  of  gases,  220; 

of  solids,  218 
Conductors,   406  ;    bad,    218  ;    good, 

218 
Conductor,  the  earth  as  a,  486  ;  light- 

ning, 446,  490 
Congelation,  233 
Conjugate  focus,  320,  338 
Connecting  rod,  261 
Consonance,  174 
Constant  batteries,  458 
Contractile  force,  223 
Convex  lenses,  341,  342 
Convex   mirrors,    318  ;    formation   of 

images  in,  323 
Cornea,  383 
Cornet-a-piston,  192 
Corpuscular  theory,  301 
Coulomb's  balance,  405 
Cvuronne  des  tasse*,  456 
Cross-wire,  364 
Crutch,  61 

Crown-glass  lens,  360 
Crystalline,  383 
Crystallisation,  234 
Crystals,  234 

Cubical  expansion,  196,  222 
Cumulo-stratus,  281 
Cumulus,  281 
Cupping,  132 
Current,   electricity,  453  ;    terrestrial, 

481  ;  thermo-electric,  499 
Currents  on  currents,  reciprocal  actions 

of,  476  ;    upon  magnets,  action  of, 

470  ;  action  of  magnets  and  of  the 

earth  on,  472  ;    induction  by,  493  ; 

magnetisation  by,  475  ;    properties 

of  induced,  495 
Curvature,  85 
Curvilinear  motion,  15 


P)AGUERREOTYPE,  379 

-1—  '     Damper  ot  piano,  183 

Dancing  puppets,  419 

Daniell's   battery,    458,  459  ;    hygro- 

meter, 273 

Dark  lines  of  the  spectrum,  349 
Debuscope,  317 
Declination,  394 


ELE 

Decomposed  force,  26 

Decomposition  of  light,  346  ;  water, 
465 

Degrees,  Fahrenheit,  200 ;  Reaumur 
200 

De  la  Rive's  experiments,  498 

Deliquescent  salts,  272 

Density  2,  of  gases,  229 

Despretz's  experiment,  463 

Developer,  339 

Dew,  284  ;  point,  273 

Dial  telegraph,  483 

Diamond,  326 

Diathermaneity,  216 

Differential  thermometer,  203 

Diffused  light,  310 

Digester,  246 

Diorama,  381 

Dip,  magnetic,  396 

Dipping  needle,  396 

Discharge,  slow  and  instantaneous,  426 

Discharger,  universal,  434 

Discharging  rod,  426 

Dispersion  of  light,  346 

Dissolving  views,  373 

Dissonance,  174 

Distance  of  distinct  vision,  384 

Distillation,  254 

Divisibility,  8 

Dominant  chords,  174 

Dove's  law  of  rotation  of  winds,  290 

Double  action  machine,  260  ;  descrip- 
tion of,  261 

Drum,  157 

Ductility,  72 

Dynanometer,  23 


EAR  trumpet,  168 
Earth  currents,  447 
Earth,  flattening  of  at  the  poles,  31  ; 

radius  of,  31  ;  as  a  conductor,  486 
Ebullition,  243  ;  laws  of,  244,  245 
Eccentric,  262 
Echelon,  345 
Echoes,  164 
Egg,  electric,  421 
Elastic  force  of  aqueous  vapour,  247  ; 

fluids,  109 
Elasticity,  12,  13 
Electric   batteries,    429  ;     egg,    421  ; 

spark,  416,  434 ;  telegraphs,  483 
Electric    discharge,   effects    of,    431  ; 

phenomena  of,  431 ;    physiological 

effects  of,  432 
Electric  light,  464 ;    stratification  of, 


536 


Index. 


ELE 


GAL 


497  ;  electric  alarum,  491;  attraction    I 
and  repulsion,   404  ;    chimes,  418  ;    j 
condensers,   425  ;    discharge,  mag-    j 
netic  effects  of,  438  ;  machine,  412  ; 
measurement   of    charge    of,    413 ; 
pendulum,  401  ;  whirl,  420 

Electrical   fluids,  hypothesis   of  two, 
403  ;    portraits,  435  ;    positive  and    | 
negative,  403 

Electricity,  399 ;  atmospheric,  440  ; 
chemical  effects  of,  437  ;  current, 
453  ;  in  chemical  actions,  452  ;  by 
friction,  law  of,  408  ;  luminous 
effects  of,  433  ;  heating  effects  of, 
434  ;  induction,  409,  411  ;  influence 
of  shape  of  body  on,  410  ;  mechani- 
cal effects  of,  436  ;  on  the  surface 
of  bodies,  409 ;  sources  of,  400  ; 
tension  of,  409 

Electrification  of  conductors,  407 

Electro-chemical  series,  466 

Electrodes,  455 

Electrodynamics,  476 

Electro-gilding,  468 

Electrolysis,  466 

Electrolyte,  466 

Electromagnets,  482 

Electro-magnetic  machines,  492 

Electrometallurgy,  467 

Electrometer,  Henley's,  413 

Electromotive  force,  454  ;  series,  454 

Electromotor,  483 

Electronegative    and    electropositive    ! 
elements,  466  ;  series,  454 

Electrophorus,  414 

Electroscopes,  401  ;  condensing,  430  ; 
gold  leaf,  415 

Electrotype,  467 

Elements,  3 

Emission  of  heat,  194 

Emission  theory,  301 

Emissive  power,  214 

Engines,  fire,  148  ;  high  pressure,  265, 
266 

Eolipyle,  259 

Epinus's  condenser,  12 

Equality  of  pressures,  76 

Equilibrium,  28  ;  of  bodies,  45,  46  ; 
of  floating  bodies,  97  ;  liquids,  84, 
87,  88,  89 

Escapement,  61  ;  wheel,  61 

Evaporation,  242  ;  cold  due  to,  249  ; 
latent  heat  of,  248 

Expansibility  of  gases,  5,  in 

Expansion,  196,  222  ;  of  gases,  cold 
produced  by,  299  ;  of  liquids,  225  ; 


real  or  apparent,   225  ;    of  solids, 

222,  223 
Extension,  6 
Eye,  structure  of,  383  ;  white,  383 


T^AHRENHEIT  degrees,  200  ;  hy- 
drometer, 103 
Falling  bodies,  laws  of,  52 
Fata  morgana,  331 
Feed  pump  and   cold  water  pump, 

260,  264 
Filtration,  10 
Finder,  364,  366 
Fire  engines,  148 
Fish,  swimming-bladder  of,  99 
Flame,  296 
Flexure,  12 
Flint  glass  lens,  360 
Float,  268 

Florentine  experiment,  9 
Fluids,     aeriform,    5  ;     elastic,     109  ; 

magnetic,  390  ;  vital,  449 
Flute,  192 
Fly  wheel,  261 
Foci,  337  ;  acoustic,  164  ;  and  images, 

34i 
Focus,  211,  318,319  ;  conjugate,  320, 

338  ;  virtual,  321,  339 
Fogs,  280 

Foot,  cubic,  104  ;  pound,  266 
Force,   centrifugal,    29 ;    direction  of, 

22  ;  pump,  147 
Forces,  22,  .23,  24,  25,  28 
Fortin's  barometer,  122 
Fountain,  Hero's,  143  ;    intermittent, 

144  ;  in  vacuo,  140 
Franklin's  lightning  conductor,   446  ; 

experiment  on  ebullition,  245  ;  kite, 

439 

Frauenhofer's  lines,  349 
Freezing  mixtures,  236 
French  units,  104 
Fresnel's  lenses,  345 
Friction,  21 
Friction,  electricity  by,  408  ;  heat  due 

to,  292  ;  wheels,  55 
Frog  current,  450 
Fulcrum,  33 
Fulgurites,  444 
Fusion,   230;    laws  of,  231;   of'ice, 

258  ;  vitreous,  230 


GALILEO'S  telescope,  363 
Gallon,  104 


Index. 


537 


GAL 

Galvanic  shock,  461 

Galvani's  experiment,  449 

Galvanometer,  473  ;  uses  of  474 

Gamut,  173 

Gases,  5,  109  ;    conductivity  of,  220  ; 

density   of,    229 ;    expansibility   of, 

109,  in  ;  laws  of  mixture  of,  136  ; 

liquefaction   of,    255  ;     permanent, 

255  ;    value   of   the   expansion    of, 

227  ;  weight  of,  112 
Gases  and  liquids,  mixture  of,  137 
Gauss   and  Weber's  electro-magnetic 

telegraph,  483 
Ghost  scenes,  382 
Glaisher's  factors,  273 
Glasses,  burning,  344 
Glasses,  weather,  128 
Globe,  luminous,  423 
Globules,  8 

Gold-leaf,  73  ;  electroscope,  415 
Gold-beater's  skin,  73 
Goniometer,  reflecting,  324 
Grain,  104 
Gramme,  104 
Graphite,  467 
Gravesande's  ring,  196 
Gravitation,  37 
Gravities,  specific,  101 
Gravity,  38,  43  ;  centre  of,  43  ;  flask, 

102,  103  ;  measurement  of  the  force 

of,  60 

Grindstone,  35 
Gulf  stream,  277 


HAIL,  286 
Hardness,  scale  of,  71 

Hammer,  water,  52 

Harmonics,  179,  189 

Harmonic  triad,  174 

Hawksbee's  air-pump,  138 

Heat,  194  ;  applications,  217  ;  atmo- 
spheres, 194;  a  condition  of  matter, 
194  ;  colour  of,  216  ;  different 
sources  of,  291  ;  form  of  motion, 
194  ;  due  to  friction,  292  ;  due  to 
pressure  and  percussion,  293  ;  gene- 
ral effects  of,  195  ;  force  of,  4';  in- 
terchange of,  209  ;  latent,  of  ice, 
232  ;  law  of  reflection  of,  210  ;  of 
water,  232  ;  lightning,  442  ;  radiant, 
206  ;  refraction  of,  331,  344  ;  from 
concave  mirrors,  211  ;  specific,  257  ; 
terrestrial,  295  ;  in  vacuo,  207 

Heaters,  267 

Heating  by  steam,  253 


ING 

Height  of  the  atmosphere,  130 

Heights  determined  by  barometer,  129 

Heliostat,  324 

Hemispheres,  Magdeburg,  116 

Henley's  electrometer,  413 

Hero's  fountain,  143 

Herschel's  telescope,  367 

Hiero's  golden  crown,  96 

High  pressure  engine,  265 

Hippocrates,  strainer  of,  10 

Hoar  frost,  284 

Hooke's  barometer,  128 

Hope's  experiments,  226 

Horn,  192 

Horse  power,  266 

Human  voice,  193 

Hydraulic  press,  83  ;  tourniquet,  79 

Hydrofluoric  acid,  200 

Hydrometers,  103,  105,  io5  ;  Nichol- 
son's, 102 

Hydrostatic  balance,  96,  102,  103  ; 
paradox,  81  \ 

Hydrostatics,  74 

Hydrogen,  density  of,  229 

Hydrometric  state  of  air,  273 

Hygrometry,  271 

Hygroscopes  and  hygrometers,  272 

Hyponitrous  acid,  460 


T  CEBERGS,  233 

•*•  Ice  calorimeter,  258  ;  expansive 
force  of,  233  ;  fusion  of,  258  ;  latent 
heat  of,  232  ;  machine,  236 

Images,  312  ;  accidental,  356  ;  and 
foci,  343  ;  in  concave  mirrors,  322  ; 
multiple,  315  ;  real,  322,  341  ;  and 
virtual,  314,  342  ;  reversed,  316  ; 
symmetrical,  314 

Imbibition,  69 

Immersed  bodies,  equilibrium  of,  97 

Impenetrability,  7 

Impermeable  strata,  94 

Incidence,  angle  of,  210,  325 

Inclination  compass,  396  ;  magnetic, 
396 

Inclined  plane,  53  ;  bodies  falling  by,  54 

Indicator,  483,  488 

Indium,  352 

Induced  currents,  properties  of,  495 

Induction,  coils,  496  ;  by  the  earth, 
494  ;  by  currents,  493  ;  electricity 
by,  411  ;  magnetic,  391  ;  by  mag- 
nets, 392,  494 

Inertia,  18,  19 

Ingenhousz's  apparatus,  218 


533 


Index. 


Kite,  Franklin's,  439 
Knife-edge,  48 

T   ACTOMETER,  108 

J— '     Land  breeze,  289 

Latent  heat,  232  ;  of  vapours,  248 

Lateral  pressures,  79 

Latitude,  influence  of  on  temperature, 
276 

Lavoisier  and  Laplace's  ice  calori- 
meter, 258 

Laws  of  falling  bodies,  52  ;  of  radia- 
tion, 207 

Leclanche's  battery,  461 

Length,  unit  of,  23 

I^enses,  335  ;  achromatic,  350  ;  crown 
glass,  360  ;  different  kinds  of,  335  ; 
flint  glass,  360  ;  principal  axis  of, 
336 ;  double  convex,  340  ;  real 
images  in,  341 

Leslie's  cube,  212  ;  differential  ther- 
mometer, 203 

Levelling  staff,  90 

Level  of  liquids,  85,  86  ;  spirit,  91 

Levers,  33  ;  applications  of,  35  ;  arms 
of,  33  ;  effect  of,  34 

Leyden  jar,  428 

Lift-pumps,  146 

Lightning,  439,  442  ;  effects  of,  444  ; 
ascending,  444  ;  conductor,  446,450 


INK 

Inkstand,  syphon,  145 
Instruments,  mouth,  192  ;    wind,  192 
Insulating  bodies,  407  ;  stool,  417 
Intermittent  springs,  150 
Intervals,  174 
Iris.  383 
Irradiation,  357 
Isochimenal  lines,  278 
Isochronism,  58 
Isoclinic  lines,  396 
Isogeothermic  lines,  278 
Isotheral  lines,  278 
Isothermal  lines,  278  ;  zone,  278 
,, 

JAR,  Leyden,  428  ;  luminous,  433 
Jets  of  water,  92 
Jupiter,  306 

KAT-EIDpSCOPE,  317 
Kamsin,  289 
Kepler's  telescope,  364 
Key-note,  175 
Kilogrammetre,  266 
Kirchhoft's  and  Bunsen's  researches,    I 


MAR 

Light,  absorption  of,  303  ;  decompo- 
sition of,  346  ;  diffused,  310  ;  dis- 
persion of,  346  ;  electric,  464  ;  in- 
tensity 'of,  307  ;  propagation  of, 
304 ;  recomposition  of,  353  ;  reflec- 
tion of,  308  ;  scattered,  310;  sources 
of,  302  ;  velocity  of,  306 

Lighthouses,  345 

Line  wire,  485 

Linear  expansion,  222 

Litre,  104 

Loadstone,  387 

Long  sight,  384 

Loops,  189 

Liquefaction  of  gases,  255  ;  of  va- 
pours, 251 

Liquids,  5  ;  buoyancy  of,  80  ;  con- 
ducting power  of,  218  ;  equilibrium 
of,  84,  87  ;  expansion  of,  225  ; 
fixed,  237  ;  level  of,  85  ;  pressures 
from,  78  ;  specific  gravity  of,  103  ; 
superposed,  89  ;  volatile,  237 

Luminiferous  ether,  301 

Luminous  globe  and  tube,  423 


MACHINE,  32  ;  weighing,  51 
Mackarel  sky,  281 

Magdeburg  hemispheres,  116 

Magic  lantern,  371  ;  pane,  422 
!  Magnetic  attraction  and  repulsion, 
389  ;  batteries,  398  ;  dip,  396 ; 
equator,  396 ;  effects  of  electrical 
discharge,  438  ;  fluids,  390  ;  in- 
duction, 391 ;  meridian,  394 ;  needle, 
387,  389,  471  ;  poles,  396  ;  sub- 
stances, 391 

Magnetisation  of  the  earth.  397  ;  by 
currents,  475  ;  limit  of,  480  ;  by 
magnets,  397  ;  by  touch,  397 

Magnetism,  Ampere's  theory  of,  480  ; 
and  electricity,  469 

Magnets,  action  of  current  upon,  470 ; 
of  the  earth  on,  393  ;  consequent 
points  of,  388  ;  distribution  of 
magnetic  force  in,  388 ;  in  the 
earth,  472  ;  induction  by,  494 ;  in- 
fluence of,  in  magnetic  substances, 
391  ;  natural  and  artificial,  387 

Major  chord,  174  ;  semitone,  174 ; 
tone,  174 

Malleability,  73 

Manhole,  267 

Manometer,  134  ;  compressed  air, 
134  ;  open  air,  134 

Mares'  tails,  281 


Index. 


539 


MAR 

Mariners'  compass,  395 

Mariotte's  law,  133 

Marloye's  harp,  184 

Mason's  apparatus,  81 

Mass,  2 

Matter,  2 

M  aximum  density  of  water,  226 

Mean  time,  344  ;  temperature,  275 

Mechanics,  32 

Melloni's  thermomultiplier,  501 

Memoria  technica,  471 

Mercurial  thermometers,  198,  202 

Mercury  frozen,  250 

Meridian,  magnetic,  394  ;  quadrant 
of,  104 

Metalloids,  3 

Metals,  3 

Meteorology,  274 

Metre,  104 

Metronome,  62 

Microscopes,  368  ;  compound,  369  ; 
origin  and  use  of,  370  ;  photo- 
electrical,  374  ;  solar,  375 

Minimum  thermometer,  204 

Minor  chord,  174 ;  semitone,  175 ; 
tone,  174 

Mirage,  331 

Mirrors,  312  ;  applications  of,  324  ; 
burning,  211  ;  concave,  211,  318  ; 
plane,  313  ;  spherical,  318 

Mists,  280 

Mixtures,  method  of,  258 

Mobile  equilibrium  of  temperature,  209 

Molecular  attraction,  4  ;  forces,  4 

Molecules,  4 

Monochord,  182 

Monsoon,  289 

Morse's  alphabet,  488  ;  key  and  re- 
ceiving instrument,  488  ;  telegraph, 
484,  487 

Motion,  14;  accelerated,  17  ;  uniform, 
16  ;  uniformly  accelerated,  17  ;  re- 
tarded, 17 

Motor,  32 

Mouth  instruments,  186, 192;  piece,  185 

Multiplier,  473 

Multiple  echoes,  164;  images,  315,  317 

Muschenbrock's  Leyden  jar,  428 

Musical  boxes,  184  ;  compound  tones, 
179  ;  intervals,  174  ;  scale,  173  ; 
sound,  168  ;  temperament,  176 

Myopy,  384 

NASCENT  state,  63 
Needle,    dipping,    396 ;    mag- 
netic, 387  ;  marked  end  of,  389 


PHO 

Newcomen  and  Cowley's  fire-pump, 
259  ;  single  action  machine,  266 

Newton's  disc,  353  ;  telescope,  366  ; 
theory  on  light  and  colour,  353 

Nicholson's  hydrometer,  102 

Nimbus,  281 

Nitrogen,  no  ;  density  of,  229 

Nobili's  thermo-electric  pile,  501 

Nodes  and  loops,  189 

Noise,  169 

Non-conductors,  406 

Notes,  fixed  and  variable,  192 

Nut-crackers,  35 

/^VCCULTATION  of  Jupiter,  306 

\J     Oersted's  discovery,  470 

Open  pipes,  laws  of,  189 

Optic  nerve,  383 

Optical  centre,  336  ;  instruments,  362- 
386 

Orbits,  37 

Organ  pipes,  192 

Oscillating  motion,  56 

Oscillation,  156  ;  amplitude  of,  56  ; 
of  pendulum,  104 

Otto  von  Guericke's  air-pump,  138  ; 
electrical  machine,  412  ;  hemi- 
spheres, 116 

Outcrop,  94 

Overtones,  179 

Oxygen,  no  ;  density  of,  229 

Ozone,  437,  444 

PALLETS,  6 1 

Pandean  pipe,  192 

Papin's  digester,  246 

Parachute,  154 

Paradox,  hydrostatical,  81 

Parallelogram,  260;  offerees,  25,  26 

Pascal's  experiment,  82  ;  on  atmo- 
spheric pressure,  118  ;  law,  76,  77 

Pedal,  183 

Pendulum,  application  of,  to  clocks, 
61  ;  compensation,  224  ;  compound, 
57  ;  electrical,  401  ;  simple,  56,  57  ; 
verification  of  laws  of,  59 

Penumbra,  305 

Percussion,  heat  due  to,  293 

Periscopic  glasses,  384 

Permeable  strata,  94 

Perturbations,  394 

Phantasmagoria,  372 

Phial  of  four  elements,  89 

Phosphorescence,  spontaneous,  302 

Photo-electrical  microscope,  374 


540 


Index. 


PHO 

Photography,  379 

Photometer,  307 

Physiological  effects  of  the  electric 
discharge,  432,  462 

Piano,  183 

Piezometer,  75 

Pisa,  Tower  of,  47 

Pitch,  170  ;  concert,  177  ;  pipe,  191 

Plane,  inclined,  53  ;  mirrors,  313 

Plumb-line,  41 

Pluviometer,  283 

Pneumatic  syringe,  293 

Points,  power  oi,  410 

Polarity,  austral  and  boreal,  393 

Poles,  flattening  of  the  earth  at,  31  ; 
of  the  magnet,  388  ;  magnetic,  396 

Poles  and  electrodes,  455 

Polyorama,  373 

Pores,  4,  9 

Porosity,  9 

Positive  electrical  fluid,  403  ;  on  glass, 
380  ;  plate,  454 

Pound  avoirdupois,  104 

Powers,  20 

Presbytism,  384 

Presbyoptic,  384 

Press,  hydraulic,  83 

Pressure,  atmospheric,  114 ;  conden- 
sation by,  251  ;  equality  of,  76 ; 
heat  due  to,  293  ;  horizontal,  78  ; 
of  an  atmosphere,  131,  244  ;  on  a 
liquid,  245  ;  on  a  body  in  a  liquid, 
95  ;  supported  by  a  man,  132 

Primary  coil,  493  ;  tones,  179 

Prism,  333  ;  paths  of  rays  in,  334 

Proof  plane,  410 

Propagation  of  light,  304  ;  of  sound, 
157,  160 

Psychrometer,  273 

Pumps,  138,  142,  146,  147 

Pupil,  383 

Pyrheliometer,  294 

Pyrometers,  205 

/QUADRANT  electrometer,  413 
\£     Quadrant  of  the  meridian,  104 

RADIANT  heat,  206,  208 
Radiating  powers,  215 
Radiation,  laws  of,  207  ;  solar,  294 
Radius  of  the  earth,  31 
Rain,  283  ;  gauge,  282 
Rainbow,  358 

Ramsden's  electrical  machine,  412 
Rarefaction,  measurement  of,  139 


SCA 

Ray,  incident,  210,  308,  325  ;  re- 
flected, 308,  325 

Real  expansion,  225 

Real  image,  322  ;  in  .-double  convex 
lenses,  341  ;  and  virtual  images,  314 

Reaumur  degree,  200  ;  scale,  200 

Receiver,  138,  255  ;  air-pump,  138 

Recomposition  of  white  light,  353 

Reed  instruments,  187 

Reflected  ray,  308,  325 

Reflecting  telescopes,  365 

Reflection,  angle  of,  210 ;  of  heat, 
210,  211  ;  internal,  330;  of  light, 
308  ;  from  transparent  bodies,  316  ; 
regular,  310  ;  irregular,  310 ;  of 
sound,  163  ;  specular,  310 

Refracting  substances,  327 

Refraction,  325  ;  angle  of,  325  ; 
change  of,  to  reflection,  330  ;  ex- 
perimental proofs  of,  328  ;  of  heat, 
331,  344  ;  laws  of,  326  ;  limit  of,  330  ; 
various  effects  of,  329 ;  through 
prisms  and  lenses,  332-340 

Refractory  substances,  230 

Refrangibility,  aberration  of,  359 

Regulator,  260,  263 

Relay,  491 

Repulsion,  magnetic,  389 

Resistances,  20,  33 

Resonance,  164 ;  box,  177 ;  globes, 
179  ;  of  air,  178 

Resultant  forces,  24 

Rest,  14 

Retina,  383 

Return  shock,  445 

Reversed  image,  316 

Rheometer,  473 

Rime,  284 

Rope  dancing,  47 

Rotation  of  winds,  290 

Rubbers,  413 

Rubidium,  352 

Ruhmkorff's  coil,  496,  497 

Rumford's  calorimeter,  297 

Rutherford's  thermometers,  204 


CACCHAROMETER,  105 

*^     Safety-valve,  246,  269  ;   whistle, 

270 
Salts,    deliquescent,    251,    272 ;   from 

sea  water,  242 
Savart's  apparatus,  172 
Scale  of  thermometer,  200 
Scale-pans,  48 
Scattered  light,  310 


Index. 


541 


SCH 

Schweigger's  galvanometer,  473 

Scissors,  35 

Sclerotica,  383 

Sea  breeze,  289 

Secondary  coil,  493 

Secondary  currents,   457  ;    rainbow, 

Seconds  pendulum,  61 

Secular  variations,  394 

Semiconductors,  406 

Semitones,  175 

Shadow,  305  ;  geometrical,  305 

Shaft,  horizontal,  261        « 

Short  sight,  384 

Single-action  machine,  260 

Sirocco,  289 

Sleet,  285 

Slide  valve,  262 

Slow  discharge,  426 

Snow,  285 

Solar  microscope,  375  ;  radiation,  294; 
spectrum,  346  ;  time,  344 

Solenoids,  477,  478,  479 

Solids,   5  ;   conductivity  of,  218  ;   ex- 
pansion of,  222,  223;  specific  gravity 
of,  101,  103 
.Solidification,  233 

Solution,  235 

Sonometer,  182 

Sonorous  body,  156 

Sound,  155  ;  intensity  of,  165  ;  limit 
of,  172 ;  post,  183  ;  in  pipes,  185  ; 
propagation  of,  157  ;  reflection  of, 
163  ;  transmission  of,  166  ;  not  pro- 
pagated in  vacuo,  159  ;  velocity  of, 
161  ;  in  gases,  161 ;  waves,  157 

Soutter's  lens,  345 

Spark,  electrical,  416 

Speaking  trumpet,  167  ;  tubes,  166 

Specific  gravities,  101  ;  tables  of,  104 

Specific  gravity,  229  ;  flask,  102,  103  ; 
of  liquids,  103  ;  properties  of  bodies, 
6  ;  of  solids,  102,  103 

Specific  heat,  257 ;  of  solids  and  li- 
quids, 258  ;  table  of,  258 

Spectacles,  370 

Spectroscope,  351,  352 

Spectrum,  346  ;  analysis,  350  ;  colours 
°f>  347  I  effects  of,  348  ;  dark  lines 
of,  349 

Specular  reflection,  310 

Spherical  aberration,  361  ;  mirrors, 
318 

Spirit-level,  91 

Springs,  93  ;  intermittent,  150 

Staubbach,  52 


TON 

Steam  boiler,  267  ;  engine,  259,  265 

Steel,  397 

Steelyard,  23 

Stereoscope,  386 

Stethoscope,  166 

Stills,  254 

Stool,  insulating,  417 

Strata-permeable,  94 

Stratification  of  the  electric  light,  497 

Stratus  clouds,  281 

St.  Elmo's  fire,  448* 

Streams,  93 

Stringed  instruments,  183 

Strings,  transverse  vibrations  of,  180 

Structure  of  the  eye,  383 

Sub-dominant  chords,  174 

Submarine  wire,  485 

Subterranean  wire,  485 

Suction  pump,  146 

Superficial  expansion,  196 

Surface,  6  ;  atmospheric  pressure  on, 
119 

Suspension,  axis  of,  48 

Swimming,  100  ;  bladder  of  fishes,  99 

Symmer's  hypothesis,  403 

Syphon,  149 ;  barometer,  123  ;  ink- 
stand, 145 

Syren,  171 

Syringe,  pneumatic,  293 

'"TANTALUS'S  cup,  150 
-*•       Telegraph   dial,    483  ;    electric, 

483  ;  line  wire,  485  ;  Morse's,  487 
Telescopes,  362-377 
Tempered  steel,  387 
Temperature,   197  ;  of  the  air,  276  ; 

mobile  equilibrium  of,  210 
Tenacity,  70 
Tension,  12  ;  of  gases,  109 ;  maximum, 

240  ;  of  vapours,  240  ;  of  electricity, 

409 
Terrestrial  current,   481  ;  heat,    295  ; 

telescope,  366 
Thallium,  352 
Thermal  unit,  256 

Thermoelectric  pile,  501  ;  series,  500 
Thermo-electricity,  499 
Thermometers,     198  ;    alcohol,     201  ; 

differential,  203  ;  graduation  of,  199  ; 

mercurial,  198  ;  scale  of,  199,  200 
Thermo-multipliers,  501 
Thunder  and  lightning,  439,  443 
Timbre,  170 
Time,  344 

Tone,  major  and  minor,  174 
Tonic  chords,  174 


542 


Index. 


TOR 

Torricelli's  experiment,  117  ;  vacuum, 

124 

Torsion,  12 

Tourniquet,  hydraulic,  79 
Tower  of  Pisa,  47  ;  of  Bologna,  47 
Traction,  23 
Trade  winds,  289 
Translucent  bodies,  303 
Transparent  bodies,  303  ;  colours  of, 

355  ;  reflection  from,  317 
Triangle,  184 
Trombone,  192 
True  time,  344 

Trumpet,  speaking,  167  ;  ear,  168 
Tube,  graduation  of,  121  ;  luminous, 

423  ;  Mariotte's,  133  ;  speaking,  166 
Tuning-fork,  177,  184;  normal,  177 
Turning-table,  29 
Tympanum,  157 


UNDULATION  of  heat,  194 
Undulatory  theory,  301 
Unison,  174 

Unit  of  length,  23  ;  thermal,  256 
Units,  British,  104 ;  French,  10 
Universal  discharger,  434 


~\  VACUUM,  124,  141  ;  formation  of 
*  vapours  in,  239 

Valve  chest,  262  ;  slide,  262  ;  safety, 
269 

Vane,  electrical,  420 

Vapour,  109;  quantity  which  saturates 
a  space,  241  ;  latent  heat  of,  248 

Vapours,  237  ;  elastic  force,  238  ;  for- 
mation in  a  vacuum,  239,  240  ;  li- 
quefaction of,  251 

Variable  winds,  289 

Variations,  barometric,  125 

Velocity,  16  ;  of  falling  bodies,  52  ; 
of  sound,  161  ;  of  light,  306 

Ventral  segment,  189 

Vertical  lines,  40  ;  pressure,  80 

Vesicular  vapours,  281 

Vibration,  156 

Vibrations  of  strings,  180 ;  laws  of, 
181;  in  pipes,  190  ;  rods,  183 

Virtual  focus,  321,  339,  343  ;  images, 
314,  322,  342 


ZON 

Vision,  mechanism  of,  383 ;  distance 
of  distinct,  384  ;  binocular,  385 

Vital  fluid,  449 

Vitreous  fusion,  230  ;  humour,  383 

Voice,  human,  193 

Volatile  liquids,  237 

Voltaic  arc,  464  ;  battery,  456,  462  ; 
couple,  451,  454  ;  current,  456  ; 
pile,  451,  456 

Volta  s  cannon,  424  ;  condensing  elec- 
troscope, 450  ;  fundamental  experi- 
ment, 450 

Volume,  6,  MOI 

Vowel  sounds,  216 


"\17ATER,  decomposition  of,   465  ; 
»  »      hammer,  52  ;  jets  of,  92  ;  latent 

heat  of,  232  ;  level,  90  ;  maximum 

density  of,  226  ;  and  mercury  frozen 

in  a  vacuum,  250 
Watt's  steam  engine,  260 
Wave,  condensed,  157  ;  rarefied,  157 
Weather,  127  ;  glasses,  128 
Weighing   machines,    51 ;  method   of 

double,  50 
Weight  of  the  air,  113;  of  a  body,  39, 

42  ;  of  gases,  112  ;  of  liquids,  78 
Wells,  93 

Wet-bulb,  hygrometer,  273 
Wheel  barometer,   128  ;  escapement, 

61  ;  fly,  261  ;  friction,  55 
Whirl,  electrical,  420 
Whistle,  safety,  270 
White  light,  346  ;  recomposition  of, 


/L53 


Whitworth's  shells,  293 

Winds,  287,  288  ;  law  of  rotation  of, 

290  ;  variable,  289 
Wind     instruments,     192  ;     channel, 

187  ;  chest,  188 
Wine  tester,  131 
Wollaston's  battery,  456 


Y 


ARD,  104 


;INC  carbon  battery,  460 
Zone,  isothermal,  278 


